The Jag v12 is an awesome engine with a fairly flat torque curve, and lots of torque. I had contemplated swaping one into a Jeep cj. Overall, if you address some minor design flaws and the coolong, they are very reljable engines, probably the most reliable production v12 ever made.. servicing is straight forward and easy if you have tge room, such as in your application, not so much stuffed in an XJ-S.

Other things to note, although all aluminum, the beast weighs in around 800 lbs. Also the earlier 75-82 engines are low compression 7.5 to 8.5:1, the HE engines 82-93? Are 10.5:1 compression but do run on 89 octane and are stout as well as easier to find stateside. The later 6.0 liter engines are also quite good. The transmission is a GM400 in the earlier HE cars, and GM 460le? In the 6.0 l cars. Both transmissions use a Jag specific case, so an off the shelf GM pattern box will not work. Chad Bolles, a reference in Kirby Palms book, does make and sell a jag to gm adapter that will allow tranny swaps to gm boxes. Also the Driven Man sells Tremec TKO 5-speed conversion kits, you could possibly source the bellhousing and tranny from him.
Fun project, but it is a project, if you have troubles sourcing an adapter, I have one of Chads originals, PM me, and I can see if I can dig it up

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)