OK, finally got it.
Seems the bearings were not moving on the hub.
Still had to add .014" to my thickest spacer on one side, via shims I made.
Cannot wait several days and pay $20.00 - $30.00+ each for the "Correct" parts.
For this project Correct is what works.

Looks like my "Spare" hub carriers are 64-67 type with the larger late type outer bearing and flat-face hubs.
They are in excellent condition and significantly lighter than '68 up versions, but I can easily see how the later versions are reinforced.
I should be able to use the later hubs with the centering ring in the earlier carriers.
So I have mixed feelings, I like the lighter unsprung weight but intend to drive the about 400+ HP and 3K Lb. weight El Camino pretty hard on occasion.

Jag was 265 HP/283 Tq. and 2500 Lb's.

Looked at feeBay and all the carriers listed are junk, cracked, welded and not machined, or rusty for spline drive wheels. The carriers I am abandoning are actually better!

Since tool J15 is unavailable I think I may use TWO spacers then do the subtraction as if I did have the J15 tool.
I see no reason this should not work and it's simpler than fabricating a precision .150 thick spacer as I do not have a surface grinder.

Found the listing for Everyday XJ, will probably contact him when funds show up.

Bit of an unhappy update.
Found tonight that both of the late model XJ6 uprights I had intended to use are damaged.
Some neanderthal used a hammer on the bosses for the lower pivot, distorting the seal races pretty badly.
Really doubt then can be hammered back to round and true.
Suppose I should have known better than to buy a rear end from an XJ6 that had a 350 crammed into it.
When I got it home the R&P were trashed too. Not my best deal. 

This was a genuine PITA.
The race that the outer oil seal rides on was loose, so the seal was not effective.
In order to tighten it up I had to remove the bearing which appears to perhaps not be intended for replacement?
There are no holes provided to push it off and it is a very tight fit.
I would up destroying the bearing cage in order to get the inner race off the stub axle.
With the outer cage removed I could barely drive it off.
Really do not see a good way to tighten up the seal race beyond Loc-Tite.
New "Do not sell me a Chinese part" bearing is on order at $44.00

Rebuilding Mom's XJ6 rear is being good practice for my own pending custom build.

Merkur based conversion has been done before, it is what I intend to use in my 'Trick-6' GT-6 build.
TRIump-buiCK-6.
231 V6 and T5, lots of frame mods.
I cannot imagine a 460 in a GT-6 or Spitfire unless you are just putting the skin on a custom frame like a Funny-Car dragster?

Let me see if I can post a pic now.

[url]100_7264 by Richard Brengman, on Flickr[/url]

Picked up my custom Curt Hamilton built Jag center section today.3.92 gears, Power-Lock posi, Wilwood calipers and and parking brake.Sorry, no chrome, but did have the rotors plated to resist rust.Pics as soon as I find my camera.

Update: My diff and carfrier parts will arrive at Hamilton Automotive in the morning. 
Decided to go with 3.92 gears.
Not a cheap build, I will have as much into this rear as I originally paid for the entire car!

Finally got to speak with Mr. Hamilton and will be sending him my parts to assemble.
Not cheap, but not as expensive as I had feared.
Seems my factory Jag 4.27 R&P are unicorn horn, he's been at this a LONG time and has never seen a set before.
So My El Camino will have an IRS nearly as expensive as the engine once everything is added up.
4.27 gears with Trac-Loc and new everything that could possibly need replacing.
He will also set up the lower arm mounts for proper parallel.

He's 84 years old, so if anyone else wants truly expert service at a pretty good price they should probably not wait too long?
 

Looking for the shop in So-Cal that does Jag rear rebuilding!
Owned by an older guy who does not do the 'Web thing.
He's been at it for decades and has parts no one else does!
Spoke to him last week before I took a long trip but cannot find the number now.
Easily found a reference to him on this site last week, but today I cannot get any clue as to the name of his company or contact information.
Just cannot seem to find the right "Magic word" to put in search.

There is a LOT of important/useful information on this site but finding it can be very difficult as it often requires searching so many threads.
I would like to see a "Resources" thread that just list contact info and basic product information for vendors.

I need to send him three center sections to be worked on ASAP, please respond if you know who I am looking for.

Huh?
The cops/legislature in PA are so power mad that this is a real question?
How would they justify such a level of intrusiveness?
Not to get too political, but are Americans now so cowering that we would accept this level of Gov. dominance?

Seems the old saw, "It's easier to seek forgiveness than permission" might apply here?
Sorry to rant rather than answer your question, I just hate Gov. overreach!

From what I have been able to glean a Watts at the bottom of the upright serves as a trailing arm.
A watts at the top of the upright is an anti-hop device.

Someone will be along to correct me if I have this wrong.

Looks like 1.00" x .120 wall for my control arms.
Kind of would have liked a little thinner but I have this on hand already and it is mostly a street car.

Trying to  work out the mounting tabs, a real joy as NONE of the frame members are strait, parallel, or level on a GT-6!
Makes reference points and measurement a wee bit difficult.

Using some rectangular box steel for simplicity in keeping the tabs parallel and properly spaced.
Looks like I will also use the old tabs to create my datum line.
Had considered mounting the arms a little lower than original but that might put them in reach of speed bumps at the frame end.
 

Closing in om my design.
Looks like I will be shortening the aluminum uprights I bought.
From 10" bolt to bolt vertically to 8.5".
This will also reduce the kick at the top around .75", need to fixture them for better measurements.

Still wrestling with weather or not to use equal length upper and lower arms?
Upper mount can be whatever I need, lower is pretty well dictated by the frame width, about 9" long max.
I have some pretty good numbers using equal length arms but am not getting the type of camber change I want.

Looks like the lower shock mount will have to be to the top of the outer end of the rear arm link.
Not ideal but lets me retain the original upper mount.

Speaking of the arms, my plan is bushes at the inside and rose joints for adjustment on the outer ends.
Would actually prefer bushes at both ends but the adjustable type I would need do not appear to be available in a correct size and would be a real chore to make myself.

I did but do not like what I see as far as remaining a strong mount and clearing the drive-shaft.
I'm in Nevada, some of our roads are a bit rough and some shopping centers should have the manager beaten senseless for putting in such high speed bumps.
So ground clearance is a fairly major issue for me.

With the Watts the lowest point will be at the center of the tire diameter.
Trailing arms have to mount further forward and reach rearward from near the center.
I think the trailing arms would actually end up with more ground clearance issue than the Watts.
Trailing arms also appear to be a likely issue with my exhaust.

Next up is my frustration to date with trying to determine what shocks to buy.
Seems all the listings are for off-road buggies, 29" at ride height, 20" compressed, 40" extended!
Not even close to my need and it seems everything else is listed by make and model.
Not too helpful for CUSTOM car builds!
Koni prices so far have been insane, QA1 has no sizing information, may have to look at SPAX or GAZ?