It may be worth looking at
Snow White Hot Rod Shop - they have developed and sold Jag IRS mounting kits for uncaged kittys for decades. That having been said, keeping the original Jaguar cage and factory setup will give you the ride and handlng that Bob Knight originally designed in 1957. This used stamped steel trailing arms/radius rods with large diameter rubber bushings and bolts into the unibody. These had safety straps fitted - an item often left off on transplanted installation - but they were there for a reason and should be used if an OEM setup is desired.
ERA offers an outboard brake rear end option but as an fyi, the mid 1993 to end of production in mid 1996 XJS models had outboard brakes. This makes servicing a breeze. The diff ratio was 3.54 on all of these, with a low bias posi. The differential housing was modified to allow the park brake cables to be mounted on special brackets attached to the housing. A very few of thes outboard braked IRS had ventilated rotors. The IRS was 61-3/4" hub to hub so you would have to narrow the lower control arms and half shafts. You can also convert an inboard IRS by using the hub assemblies and halfshafts (narrowed of course) from a Jag XK 8 up to 2003 vintage, but you have to make your own parking brake mounting solution as the inboard diff housing does not have any bosses for this. I got some XK8 hubs, brakes and halfshafts from a Jag only wrecking yard.