OK, I'll step into this mine field. 
C3 'Vette is already IRS but I've never been fond of their cart spring system.
I see several kits out to convert "Vette rear to coil-over, just like what Jag does from the factory.
For the standard 'Vette performance the stock C3 rear is probably fine.
If you have a more ambitious level of driving planned the Jag IRS will allow much easier adjustment of ride height and spring rates.
A coil over conversion of the C3 rear is probably available with no need to engineer anything if you have the $$$.
A Jag conversion will not be cheap by the time it's done either.

Your call as it's your car.

GFinally got a Merkur rear from a manual trans car.
Posted lots of "Before" pics in the import IRS pic thread.
I will need to "Uncage" it similar to what is commonly done for a Jag.
I may be able to use the center of the original cross-member diff support for my install.
I will be looking for some shorter axles, making my own control arms, and my own uprights.
At least the outer carriers are a "Bolt-On" design.
Looking for Euro-Ford bits to convert to disc brakes.
Coil over shocks are about all I will be able to just buy locally.

OKAY, finally got my Merkur rear and some pictures. 
4x108MM PCD, 60" mounting face to mounting face as best I could measure it.
Aluminum carrier with 3.64 gears and an open diff in the manual trans cars, automatic is a little taller.
Fairly heavy in stock form, still much lighter than Jag or 'Vette.
I will get weights of each component when I get it apart.
Forward cross member might make fitting fairly easy in some cars, or narrow it for others.
There is a rear two bolt mounting point on the diff cover as well, factory has a big rubber mount there.
Three mounting bolts at each side of the diff carrier so almos any sort of strong adapter should be possible.
As stock it uses unequal length axles with the common six bolt attachment, I will be looking for axles about 4" shorter per side.
Original springs sat in pockets integral to the lower control arms.
Original shocks mount to the back of the control arms.
All pretty standard and similar to most of the other modern IRS production cars I have looked at.
I always heard that these are "Rare" cars, but at this time I know of SEVEN in fairly local salvage yards.
This rear is as common as dirt in the U.K. and Europe, LOTS of goodie$ are available for it.

Mine started out filthy, begins to look fairly good once it's nicely finned aluminum castings become visible.http://www.specialinterestarms.com/DSC01055.jpg

 

Half-off at P&P for the holiday weekend. 

Got a chance to look around at a few cars they have. 

Lexus LS400 diff looks almost like it was made for exchanging into a Triumph except that it has an odd guibo type joint on the input. 
Appears to be an open diff. 

Porsche 944 trans-axle can be had for under $100. 
Interesting, but a lot of work to make it fit and deal with the torque tube. 

61 Pontiac Tempest rear complete, another interesting engineering exercise. 
Flexible drive-shaft (Really!) and an American made trans-axle intended for a front mounted engine.
Not sure  but it may be a three-speed. 

Nissan 300 ZX diff, appears to also not be a posi but not too hard to make work. 

'96 Ford T-Bird, diff is not as large as I feared, not posi. 

CL ad has a Supra posi diff for $200.00 
Not sure of the ratio or miles but I know they have been adapted to Spit/GT6 often enough that I think there is a kit available. 

Found five Merkur's in semi-local P&P yards! 
Rear is very similar to T-bird but a bit lighter. 
Also found an Escort GT twin cam parts car locally, not sure if the disc brakes can be adapted to the Merkur hubs? 

So I have until monday to decide what to use for my GT6/Buick V6 and T5 IRS rear build.

I do not have the old calipers for the 3.54 diff and they are different than the newer carriers.
I need to find someone to put the posi from the 3.54 into the 3.31 case.

327 is s shorter stroke than a 350, less piston speed and free revving.
To build a 7000+ rpm 350 is big money, 327 will cost less and live longer.

Found some refernce points and measured the center of the arch for the axle from the old frame and live axle.
Now looking at the Watts link, looks like I only have about 3" available end to end since I would not want it on the ground if I get a flat tire.
Tire sidewall is only 5".
Is this enough length?
I could possibly push it as far as 4" end to end if I must.
It also seems that the Watts arms should both be at the same angle and length to work properly.

With my lower suspension arms set level I get about five degrees on the axles.
I know they need some angle for the U-Joints to work properly, is this enough?

I am surprised to see how low several suspension components will sit, bottom of the watts link and the diff in particular.
I will have to measure the low points of the old live axle for comparison.
I've had issues with the front of my frame and "Speed Bumps" in the past.
Clearly no off-road action with this suspension and tires!

Getting close to beginning to weld!
Need a little bit of flat plate and to verify my design.
Tomorrow I will pull the rear wheels to make measurements from the original El Camino axle.
I have the GM chassis book but cannot find anything stating exactly where the axle sits in the frame arch.
I will also make some measurments from an XJ6 for diff height.

What I need some help with is setting pinion angle.
It would seem to me that I want the lower arms to sit flat.
But they are mounted to the pinion housing.
Not sure how much, if any, angle might be desireable?
I thought I saw some discussion of this on the forum but my recent search did not find it.
Now also wondering if I dare make the rear mount without mocking up an engine and transmission first?

Tempted to weld in the upper mount for added frame rigidity but thinking that even a bolt-in design will help if I use poly bushings.

A little concerned that the lower watts link may be fairly close to the ground, need to get the correct lug nuts to mount my wheels for a measurement.
Watts link pivoting arm will have to be fairly short.
Trailing arms might be simpler but get in the way of the exhaust system.

Lots of interesting pictures here, http://sideways-technologies.co.uk/forums/index.php/topic/1228-jangos-rear-end-kit/

GT6/Spitfire true IRS conversion using the Merkur/Sierra center section.
Looks like it was a success, then dropped out of sight, why?

I'm trying to contact the maker for more info.

Quick pic of a rough mock-up.
Not the actual steel just some pieces I have laying around.
"K" shaped top mount.

There is no reason why a custom mount cannot be a drop-out type.
I am putting Jag IRS into a '68 El Camino and it will be a custom mounted removeable set up.
Start with your frame width, compare to Jag, look at your ride height, wheel diameter, etc.
Even if you keep the cage there will be some custom mounting required.
Several threads here on doing this, I know the Jag into Chevy truck is popular so you may find a thread with you exact conversion.

After MUCH searching I finally located a couple of useful Merkur IRS pics.
About 58" track when stock so still a bit wide for my use.
But also similar to the T-Bird but a bit lighter and fairly strong, seems to be good for about 300 HP in a light car.
Since it's English Ford/Sierra there are tons of LSD's and orther upgrades available.
Seems to be a fairly common swap into older cars in the U.K.

 

Most from this site, http://forum.merkurclub.com/forum/viewtopic.php?f=37&t=24193

Oh, and on the Trick-6, I'm now looking for Sierra/Merkur IRS pics and info.
Has the right PCD for my wheels and I'm pretty sure they are disc brake as well.
A bit rare in the U.S. but I do have a line on a couple that are just a little too far away to be convenient.

AARGH!
Still no Sierra/Merkur pics.
A bazillion made, and no pics on the web.

I now have a hazy idea of how to do my IRS for the "Trick-6". (TRIumph GT-6 + buICK V6 engine)
Of course wanting to use my 4x108 PCD 14" x 6" Enkie (Very light) wheels gets to be an additional issue.

Looks like Ford or Supra diff. will be easiest.
Both can be had with forward mounts that may be close to what I need.
Ford should be unbreakable even with an aluminum carrier, I only have a little 225 V6.
Supra may be smaller and so easier to fit.

Ford appears easier to find, in cast iron.
Supra seems to be a little hard to get, CL sellers don't answer if you live more than a couple of miles away and tend to want high prices despite not even knowing the gear ratio's.

Supra's do show up in the P&P salvage yards, IF you can get there fast enough.
If not they will be on CL a day later, at 5x the price.

Outer end may be simpler if I can use Ford, some are the correct PCD, have to verify that some IRS are still 4 x 108mm.
Also need to confirm a 14" wheel clears.

No matter what I use it's going to be custom control arms.

Found this reference on the web, from V8S2000 board:

"Ford 8.8 IRS Info
ALL 99/01/03/04 Cobra axles have a 28 Spline outer that goes into the hub

99 Cobra axles have a 28 spline inner that goes into the diff

01 Cobra's came with stronger axles than 99

01/03/04 Cobra axles have a 31 spline inner that goes into the diff

03/04 Cobra's came with stronger axles than 99/01

03/04 Cobra's came with Carbon kevlar clutch packs

03/04 Cobra's came with larger pinion flange than 99/01

All Cobra's came with limited slip

There was not a 2000 or 2002 Cobra released on the american market.


The 8.8 IRS was offered in the following vehicles

99/01/03/04 Cobra

1989-1997 Thunderbird Supercoupe (All Posi-traction)
1992-1997 Thunderbird V-8 models with rear disc brakes(some posi-traction)3.08, 3.27 
1992-1997 Lincoln Mark VIII (All aluminum carrier, most non-posi-traction) 3.07, 3.08, 3.27, 3.55, 3.73
1989-1992 Mercury Cougar XR-7, supercharged V-6 (All posi-trac

A late night update.
Found some great pics of Fiero rear suspension uprights modified to use an upper A arm.
LONG thread with CAD drawings and much discussion here, http://www.fiero.nl/forum/Forum3/HTML/000116-16.html
I am not interested in the rocker shocks and his drawings will not directly work for my use.
I probably have to build a sub-frame at the rear of the GT6 but with the body off that should not be hard.
I hope to be able to leave the wheel wells alone.
This may be a way to keep a budget build while also getting much improved rear suspension geometry and brakes.
Since I want to keep 13" wheels I expect to retain the original Fiero calipers.
Much more research to do, hoping to visit a junk yard this week with some calipers and digital camera in hand.