My '49 has a Jag suspension under it.

You can see it here-

https://67-72chevytrucks.com/vboard/showthread.php?t=687778

There is a build thread with more details on the suspension too- starts at about post 17

https://67-72chevytrucks.com/vboard/showthread.php?t=725080

If you want some good information on Jag IRS, go here-

https://dazecars.com/dazed/JaguarIRS1.html

It is Mustang focused, but some good info.

Just a few tips -

-an IRS out of a XJS will have a locking differential (Powerloc), but XJ6s are usually an open diff
-An IRS out of a XJS with the straight six (93+?) will have lower gear ratios good for and OD transmission, otherwise you'll likely end up with a 2.88
-I've never paid more than $300 for a Jag IRS