My brother in law has just put a Jaguar v12 in his Trans Am. It drives lovely and goes well; but sounds wrong. The engine should have stayed in a Jag.
I am truly heartbroken. The "mechanic" you went to was a sorry example. In my experience, only neglect will kill these engines. They are more durable than any other I can think of unless over-heated (due to cooling system neglect). Over-heat them (or any other aluminum engine) and you'll drop a valve seat or loose a head gasket, but certainly not the end of the world (sure, it's smack-you-up-side-the-head dumb...). These engines are completely conventional for the purpose of any engine work you would need done. I wish you had the benefit of more experience with this car before the failure. There is no "V-12 Boogyman". The XJS was designed around this great engine. The G.M. transmission is another story.
I love American V8 engines, but I would NEVER put one of them in a Jaguar XJS, or any other Jaguar for that matter. Usually those who lack knowledge or are cheap with their wallet get convinced that the problem is the V12, when in fact it is both mechanics and Jaguar owners that are the issue.
My 1984 Jag V12 is the coolest car I have ever owned. I love working on this car when needed. I do think there are only 368 that were made in the world. I wonder how many are left?
I just bought my 84 Jag XJS V12 5.3 HE, and love it. Sure it has some small quirks to work out, but nothing major. What gets me is the people who buy one, and then put the car down for some reason or another, but let's remember the time they were built. Think about it, some of the things that go wrong or go out, happen to other maker's cars.