Too bad you couldn't put EFI with throttle bodies like I saw in the 80s bmw M6 some guy had. He said it was doing 350hp from a six. Those jags are cool but with 12cylinders they might be deadly. My ideal would be to get one cheap and drive it till it dies, like my dad's friend who overheated one and dropped the valves. Parts shouldn't be too hard to find, I saw one last time I went to the junkyard.
In between Triumphs I had an '85 XJS that my Dad liked so much, he went out and found a '76 with factory 4-speed manual transmission. That engine is very impressive if you look at its internal specs, strong and well-built. I used to have lots of info on it, but only kept the collected road tests book and factory service manual after I sold my car. The basic idea, according to one interview in a classic car mag, was that Jag wanted to build an "everyman's" V-12 engine, and I really loved it. I would never, ever recommend that SBC conversion. There is also a pretty good 5-speed conversion that is/was on the market.
Mechanically, they are more simple than they look. Get a factory manual, and a Haynes for back-up, and you will probably be able to do most everything yourself. Oh yeah, if dropped valve seats worry you, remember that they were only an issue in the HE heads. Also, there are aftermarket seats which solved that problem. In my opinion as a previous owner, these cars have an undeserved bad reputation. Of course, my perspective may be skewed after years of British and Italian sports cars...
I have a British friend who's really worked on his XJS intake, exhaust, fuel injection, and suspension, and THAT car is really something special. If you've got the time and talent, and like the thought of a screaming V-12, it's hard to pass up an XJS.
I can't believe nobody has posted this yet!
http://www.youtube.com/watch?v=DoxEis_Y7F0
needless to say, those motors are tough!
I just sold my XJS and the engine was the best part of the car. Smooth, loads of torque, great sound and...wait for it...mine was dead-nuts reliable. I'm sure mine was the exception, but I liked that V12 so much that I began actively looking for an XJ6-C to swap the V12 into.
There's a guy in my area named Stew Jones who develops and races old V12 Jags. He also adapts manual gearboxes to them (BMW/Getrag I think). He makes some real monsters and the 12's really do sound mean!
http://www.jaguarv12etype.com/servframe.html
This is Stew Jones' website.
Everything (mechanical) that he does to V12 E-types applies to the V12 XJS...including the 600hp 7.3L V12s with Webers, the Getrag manual transmissions, etc...