Looks like I have lucked out.
The mounts from my good rear are only drilled one so all I have to do is reverse them and I may drill any angle I like.

Found my answer on the arms/axle angle question!
Whatever gives me -3/4* + or - 1/4* at normal ride height.
Strait out of my old XKE manual.
Now I need a better way to measure my camber.

One more point to be clarified. (Yah, sure, just one more!)
Browsing the links posted into varied topics here I found a mention that a Watts lint to the top of the hub serves to eliminate wheel hop, and that a Watts link to the bottom of the hub acts as a trailing arm.
Is this correct?
Doing both might be possible but seems overly complex and likely to create a binding situation.

I really want to use a Watts link but am dismayed at the limited ground clearance I will have if I mount it to the lower arm near the hub.
Some jurisdictions will ticket you for anything that sits lower than the bottom of your wheel rim.

I find other issues with a trailing arm setup weather it is forward or rear mounted.
Getting to the point I have to decide what I am doing about this.
 

Made some actual progress!
Ideal had to yield to practical in a couple of respects.

Lowered rear ride height l.00" to only 8" to the frame bottom just ahead of the kick-up.
GOOD.
Lowers center of gravity for better handling.
Helps to get my engine to sit normally for it's designed angle.
Brings the lower trailing arms for bracing the bottom of the diff into an almost strait line.
Brought my main diff mount up on the frame rails.
BAD.
Increases the bend I need for the upper forward reaching diff mount reinforcements.
Reduced compression travel to only 3", maybe 3.5"


Here is an actual mock-up of my recent progress.
Main mount will be bushed at the ends.
Shock mounts and their spreaders are tacked in place.
Mounting points for the forward facing upper reinforcement are tacked in.
Lots of clearance for brake components.
Plan is adjustable shocks on the front and air shocks to lever the ride when I have a load at the back.
The round tube behind the diff is a mock-up for possible reinforcement of the rear lower diff.
30* black bars are my fixed points for shock simulation at ride height.


I have to bend tube for the forward reinforcement.
Despite my desire for light weight I seem to be overbuilding this.
I hope the added weight at least helps to balance me out front to rear.

Welll, I lean toward as few power robbing accessories as possible.
Steering that is light around town may be twitchy at speed.
PS pump takes what, 8HP?

I may be a bit extreme about this as in addition to dumping the PS I'm also hoping to drop the PB via dual masters and a balance bar.

Pic link is dead.

Visited a local "Rat Rod" car show today, never saw so much BAD engineering in one place before.
Some of it was truly incredible, but not in a good way.

To get the images out of my head I had to work on my El Camino/JAG IRS tonight.
Liking the Watts Link but not happy with the lack of ground clearance if I do it the easy way, like this but I have 15" wheels.


This pic might be an inspiration.
I do not have space to add to the top of my upright as he did as I only have 4.50" between the top of the upright and bottom of my frame.
Need to keep 4.00" of travel.
But if I can extend the top of the upright inward 3.00" I could put the Watts above the axles and use the old trailing arm mounts for sway bar link mounts.
So how well do the Jag upright castings weld?
Thinking I may be able to cut pieces from my spare set and weld them together to get the additional width I need.
Would also weld the pivot point support at right angles crossing the top of the weld joint for a bit of reinforcement.
Hope this is clear as I've yet to master cad for illustration.
If doable this could make a clean Watts setup.

Need to stick my spare fuel tank in the frame to verify that I have plenty of clearance for the IRS, I think I do but should verify before going further.
 

Some small progress.

El Camino frame is finally INSIDE on a nearly level floor!
Got the wheels bolted on and a stand made for the center section to keep it level.
With shims I will be able to adjust the components heights.
Bought my bushings so getting closer to fabricating the mounts.

Also bought some cast aluminum uprights for the Trick-6 rear.
Originally for a South African Lotus Seven replica, I got them from Canada.
They use my Merkur/Sierra hubs and axles.
Not beautiful but functional and I think I can clean them up using my milling machine.
Might even be able to lighten them a little.

How similar to Falcon, which is basically the same as early Mustang?
Lots of Mustang info available.
 

Have no pics and not sure if they would go under Chevy or import since it's a GM car on an Opel chassis design.
Aluminum center section, semi-trailing arms, and drops out on a cradle frame so should be fairly easy to adapt.
Suspect that newer CTS is similar.

The quad-cam V6 engine is surprisingly attractive and can be mated to manual trans.
Probably a PAIN to adapt to other cars though due to excessive electronics issues.

Not directly related to the IRS but necessary in order to be able to check driveshaft length and angle.
Finalizing fitment of the engine and trans which requires heavy frame cutting.

Not actually building a frame but with the number of modifications it almost feels like it.

In order to get a Buick V6 and T5 to sit low and back in a Triumph GT6 frame I had to cut away most of the top and half the inside vertical wall of the forward half of the "Hourglass" portion.
Pics once I clean things up a bit.
Really want the bonnet to remain stock and do not want to put thick spacers under the body, it's sports car not a 4x4.
I will also be putting a curve into the top of the front cross-member to clear the crank pulley.
Crankshaft damper sits flush with the front edge of the cross member, that may give an idea how far back I'm setting the engine.

Think I figured out how to reinforce the frame.
Going to have some rectangular section steel bent to match the chassis shape where it's been cut away.
This will need to be as close to the inside width as possible and half the depth.
That will be welded inside the original chassis, which is even thinner than I had expected despite having no rust.
This restores the frame to a shallower box section.
Then I want to add another bent box section to the inside outside edge, about a third of the original width but flush to the top of the original rails.
That gives me a doubled up box section that I think will probably be stronger than the original when welded full length.
But it still leaves the extra inside width and depth I need.

Now to find a fabricator who can do the bends.

Hope my description makes sense, I have no idea how to attach a drawing even if I had one.

Also looking into changing the front IRS to eliminate the weak lower trunion.
Anyone know if 71-73 Pinto upright is actually the same as late Cortina?
I have confirmed that they are NOT the same as 74-80.
Slightly smaller and lighter, with a 4.5" arm instead of 5.0"

Had not planned to do so but put a few hours into fitting the engine and trans today.
Finally have them sitting just about as perfectly as possible.
Cannot get any lower even if I had a dry sump system, bottom of the pan is even with the bottom of the frame.
Engine is dead level with the frame, I may want to raise the front just a fraction.
Front of the crank damper is just about flush with the forward edge of the front cross-member.
If I cannot get the bonnet to fit without cutting now, it cannot be done using a Buick V6 engine..

Aghast at how much of the frame channel I've had to remove for that big fat T5 and the V6 starter.
Going to have to get very creative to restore the frame strength.
Looking like I will take a page from the old Shelby Mustang, putting a removable brace between the shock towers.Find this advisable since I've had to remove some of the original gusseting to clear the oil pump.
Also cutting clearance for the front pulley in the forward cross member.

Reversing an aluminum single sheave SBC crank pulley will give me enough room to move the steering rack back 1.00" which seems to be all that should be needed.
Doing that and shortening the rack to fix the backward Ackerman and bump steer.
No room to mount the alternator in a conventional manner so current thinking is to reverse mount it.
The water pump drive may become even more bizarre, perhaps utilizing a jack-shaft from the alternator if I cannot make an electric pump fit.

Just about ready to actually put the front engine mounts in.
Would be nice to weld parts ON after so much cutting them off.
Still not certain about the front suspension.
I recall reading that some folk have adapted European Ford spindles and arms.
Since I need a 4x108 pcd anyway I will look into that as well.

Once the engine and trans are finalized I can also finalize the mounting of my rear diff.
From there rar suspension mocking up should be much more accurate. 

Gave up on doing what some one else had done and actually made some progress today.
Mostly cutting metal but I think I've figured out how to mount the Sierra/Merkur diff!
Good thing since it was keeping me awake.
Pics once I'm a little further along.
Basically I cut away most of the original Sierr/xr4ti diff cradle except for the mounting plates and a small section of the curved tube between them.
I plan to weld some rectangular box section steel virtically to the ends terminating at horizontal brackets that will fit the original Triumph mounts.
This is similar to how Toyota and such are usually done.
In addition I also cut away the center of the rear body mount.
I will now be able to add a horizontal bar to reinforce it while also fitting an original Sierra rear diff mount.
Hoping I can get one in Poly as the original rubber is too soft for my preference.
Once done the diff will be able to drop out the bottom easily as original.
I am off-setting it a bit in order to have equal length driveshafts.

With the diff in I will be able to work on the A arms, uprights, and shock mounts.
Since this is not a sancioned racer there are no set rules and I am free to do whatever I like for suspension.

Anyone have an "Ideal" rear suspension geometry for a 49" track?
 

Bought the Speed-Wiz software since it was cheap enough and does much more than just suspension.
Found that it has Jag in the suspension menu so that should help with the El Camino.
Still pretty much stuck on the Spitfire/Merkur custom setup until I can figure out my basic starting points.
I know the track, wheel/tire sizes, and some basic differential dimensions.
Beyond that it's a blank piece of paper!
I have pics of a great design but no dimensions as the guy who was doing them insisted on his doing the  installation.
Not practical with him being in England, beside which it seems he dropped out of doing them.
So it looks like I have to use the pics to try to get rough dimensions and come up with my own version.

Biggest issue of the moment is that the stock Spit steering rack needs to go further back to improve the suspension, but cannot.
Now thinking of adapting an entirely different suspension as I need 4x108 PCD, bigger brakes, less bump-steer, and the steering rack a little further forward if possible.
Have to retain the stock 49" track width too.