I like the fact that you have moved the coil over attachments outboard.  This improves motion ratio as much as you could.  Nice work!

You probably are looking at a  Salisbury differential.  Easy way to tell is if it has a drain plug.  Regardless, Power Lock was an option.  One way to tell if you have a "posi" rear in a car is to put it in neutral and raise the rear off the ground.  If you rotate a wheel and the other wheel rotates in the same direction it has a locking feature built into the differential.  If the wheel on the opposite side rotates in the opposite direction it does not have posi-traction.  If you don't have power lock it can be added by someone like Curt Hamilton.

Numerous ratios are available for the Salisbury, if a gear change is required.  There should be a tag, attached by a cover bolt, indicating the number of teeth on the ring and pinion.

https://www.youtube.com/watch?v=T0DeTzQWNBQ

I'm in the same quandry with my '65 Mustang.  The Coyote engine does not have provision for a power robbing power steering pump.  A 20:1 manual rack seems like it will be to slow, but they are a lot more inexpensive than the quick ratio Flaming River.  Your Vicki should be fairly light in front and you can probably get away with the 15:1 manual.  Is there someone at F. R. with experience that can help you?  I found this quick ratio rack on E-Bay, http://www.ebay.com/itm/120870038809?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT , but the seller does not statet the actual ratio anywhere in his ad, that I can see.  Cheap enough to try, and we could always go to power, later. 

I see no problem narrowing the Jag rear end for your cruiser.  The IRS will ride well and is strong enough to survive the LS3 with an automatic trans, in my opinion.  There is a ton of information on this site, about narrowing the Jag IRS, tips on controlling wheel hop, radius rod how to, Watts linkage, shock valving, etc.  Curt Hamilton narrowed my XJ6 and added posi-lock because I did not want to mess with it.   For pro-street or drag racing I would not go IRS; again my opinion.  Anybody want to chime in?

Check out the Jag IRS in Bodie Stroud's '31 Plymouth project gallery.
http://bodiestroud.com/project?album=2&gallery=13

I hope people will study the effects of motion ratio when determinining spring rates.  I noticed the link in previous forum topics is broken.  This one seems to work:

http://www.sae.org/students/presentations/suspension_101_by_steve_lyman.pdf

Here is an interesting approach to anti-roll, and squat control.
http://www.youtube.com/watch?v=bbgjRBT4ltM

Don't forget motion ratio as well as spring angle.
http://www.sae.org/students/presentations/Suspension%20101%20by%20Steve%20Lyman.pdf
See slide 8.
GAZ has a selection of coilover shocks for Jaguars. http://gazsuspension.com/Gaz-Rear-Dampers.html 
For a dissertation on Motion Ration and an easy way to determine wheel rates see: http://irsforum.boardhost.com/viewtopic.php?id=336
With a Jag XKE or XJ6 the total shock/spring travel is no more than 4.00 inches.  1/2 of this available for jounce is the minimum.  I used .6 to 2/3 for my design.

The answer depends on how well you design the linkage, bellcranks and their locations.  Design intent of the Mumford was to eliminate lateral movement.   One would have to model this on paper or in CAD and look at bump, roll, ground clearance, and roll center.   There is a risk of binding with the Mumford but it should control lateral movement better than a panhard bar.

I was surprised by this number because I have been using Alden Eagle 654 data for my installation.  They used to list their 654 coilover speific to Jaguar rear.  The compressed length is 9.7 inches with a 3.7 inch stroke. 

You are in good shape as long as the shock does not bottom, or the spring does not go solid, before firmly on the bump stop.
Monroe Part No. 39003 specifications:
Jaguar XJ6
Compressed Length: 9.375 in
Dust Shield Included: Yes
Extended Length: 13.25 in
Parts Pack(s): No
Travel Length: 3.875 in

In answer to your first question, this is correct if the plane is parallel to the inner pivots of the lower control arm.

Locate the inner pivot of the radius rod on an axis through the upper inner arm pivot.  This axis should be parallel to the lower arm inner pivots both horizontally and vertically.  Looking at your pictures, you may have more room running the rod aft.  This would put the rod under tension during accelleration, which is good. 

A few inches forward won't do much good, but it would be a good start for a forward pivot of a A arm,  with the rear pivot toward the back.  The further these pivots are apart the better.

Without talking to some Ford Mustang drag racers, I don't know if the FFR shocks work or not.

The FFR parts are essentially shock absorbers.  I was hoping you could find room for a true radius rod running aft or forward. 

Poor Man's Brass Hammer
A must have tool when welding up small holes in sheet metal is a brass hammer.  This is especially true when the metal is thin.  Hold the face against the backside and mig up the hole.  I made this one with an old ball peen and a brass freeze plug. 

It looks like your question has been asked a hundred times on www.S10forum.com
User Milleraa posted a picture of his C4 installation in an S10.

It looks like the crossmember would fit between your frame rails with some simple brackets.  You will have to narrow the rear or run wheels with a deep offset from the back.  Flat Out Engineering in Orange, California may even offer a kit to install a Corvette IRS in an S10.

Hi Swinfen, 
Your pictures did not show up, but you can still edit your post to make them appear.  Go to the forum index and Day has described how this works.

Regarding your rear output seal leak, take a look at Kirby Palm's "Experience in a Book".
Below is a link to a free download.  Worthwhile reading. There are a lot of helpful tips on rear suspension maintenance.  Without pictures I don't know whether you have a Dana (no drainplu) or a Salibury (with drainplug).  Some tips on rebuilding the differential  begin on page 349.

Hope this helps.

http://www.jag-lovers.org/xj-s/book/Jaguar.html