There's an XJS convertible IRS on ebay. Since its a convert it would have the 3.54 ratio, power-lok, reinforced hubs, finned aluminium diff cover and outboard brakes. It's also basically a drop in after some plumbing for the rear brakes.

Jaguar XJS XJ 12 XJ 6 Ratrods Independent Rear Suspesion Differential Brakes | eBay

 

Its froma v12 so it would have the 2.88 diff not 3.54, the 6 cyls had the 3.54

 

Hmmm. My 83 XJ wuzza six is 2.88 equipped. With the .70 OD of the 4l60#, it is long legged. But, the torque of the LT1 and the deep 1st of the 4l60E pull it well.

It makes a degree of logic, that the torque of the 4.2 goes well with the 2.88, while the rev capability of the 6.0 goes best with the 3.54.

I've alwys struggled with the logic of a freeflow DOHC head on a long stroke lower end???

Carl