Any techs available in the Omaha, NE area that would be willing to help diagnose my XJS? Another set of eyes on a problem is always a good idea.

 

If you want to evaluate the V12 see if it even cranks over and runs on starting fluid. Even when turning over on the starter they should sound smooth and with an even rhythm. If it's got a dropped valve or some other problem it won't even sould well when cranking. As an experienced petrol head with a lot of V12 experience I can tell how they are even when cranking. Your mileage may vary.

LS swaps are great. I'm in the final stages of my latest swap. You can expect to spend 10 to 15 k to do an LS swap and make it reliable. An iron 4.8 / 5.3 with a 4-speed will be closer to 10 and anything with a six-speed automatic or 6.0L will easily eclipse 15K. Are you up for that type of investment? If so you better make sure you're starting with a good car. Nice interior straight body otherwise you could easily be at 25k all said and done.