Firebird - welcome aboard!
Sorry I can't answer the jag questions -- but theis is the right place. 
However, I'll jump in as a Vette-Mod Maniac... No mines here Richard!!!

I built a Giovanni-like Set Up - you can find it here on the site (url follows), google, or over @ vettemod.com - I use the  same handle there, phantomjock.

The Govanni is a HUGE improvement over the standard C3 IRS.  Yes, the C3 is an IRS, but the trailing arm suspension has its own set of quirks that leaves a lot of room for improvement.  Having owned my C3 for over 35 years, autocrossed and autobahned, I became well familiar with its quirks. And, I'm sure the jag IRS would be an improvement - but not without some manufacturing/mod efforts.   

You can see the difference on the curves on my build thread @ http://irsforum.boardhost.com/viewtopic.php?id=490   Similarily, the road test report I've posted there attests to the much improved handling and streetability for this approach.

My Giovanni set up uses a dual mount composite transverse leaf, and the bushings in the TAs have been replaced with sphericals, so the motion is smoother and much improved.  Parking brake is moved to the top of the TAs. 
Two aluminum plates lower the lower cross member for better geometry on the lower control arm. 

The setup includes 2 safety loops as well, all (or most) of the necessary small bits and pieces for the upper and lower control arms.  I had all the welding done to the rear differential crossmember and it is part of the package too.  The diff crossmember mounts with aluminum pucks instead of rubber/poly discs so much improved road contact as well. 
The spindles were rebuilt all new bearings by VanSteel, and they powdercoated the TAs as well. There is always a concern regarding the in/out motion of the half shafts and wear and tear on the differential.  I opted for slip shafts that were built by Chris Taton who uis highly regarded in the off road community for his slip shafts. 

The set up is a wheel to wheel one - just add brake calipers.  If this sounds like an advert - well, I guess it is - as my Giovanni is For Sale.  I haven't priced it yet - but there is a lot of all new gear there and it would be sold a package.  That is all pretty much covered in the first post at the URL i provided.  My Giovanni has never been out to the street, the bearings, etc never seen anything but the garage.  Once I completed and bench tested the Giovanni, I realized I needed to take a different track.    

For the ultimate in simplicity, this is a R&R, bolt in/plug and play set up.  You could be on the road after a good weekend in the garage. Use your differential. 

As you follow that thread, my posts will take you deeper into my personnal decent into madness with my C5 adaptation to the C3.  I owe a number of updates to that and must post those soon.  I am offering my Giovanni (as good as it is) for sale because I am now building a full-blown track car racer and require the improvements I can gain using a full-on Dual Wishbone and C5 setup.  [Also, my wife/CFO wants it out of our living room!] 

If you have interest or questions - post or pm here, vettemod, or digitalcorvettes.

Cheers - and welcome aboard.

Jim
BTW - if you want to push 1g cornering - you'd need to get better bearings than are available for the C3 spindles.  The C5 racing  bearings - good to well over 1g are about $500 each. 
 

tyrellracing wrote:

Gee I thought I was the only one here that keeps my shiny car parts in my living room. .... She claims they reproduce.LOL

Yep!  Started with the intake manifold and webers on the fireplace mantle, then a few interior parts, now the complete interior, and the IRS bits from above.  Add a rear wing, crank timing gears, a transmission cable, and so it goes. 
She's accused me of building a car in the living room - go figure?