Well I picked up the replacement engine last weekend. It came out of an 49,000 mile 86 XJS that had an engine fire due to a fuel line leak. The wires, sensors and rubber lines are toast but the engine appears to be fine. The fire wasn't hot enough to melt anything important, and I have all the wires, sensors and stuff on my 83 that it is going into.
The engine turns over nicely using a ratchet on the crank but I haven't performed a compression test yet. I have a small problem, one of the spark plugs won't come out. Someone had tried already to remove it without cleaning out the debris in the recess around it and stripped the nut. So far, using a shop vac and a small punch, I've removed all the porcelain (almost in one piece!) and have let is soak in PB Blaster (penetrating oil) for a couple days. I'm going to try heating it up with a propane torch, then douse just the steel plug with oil to cool it quickly and see if I can get it out with an easy-out.
I'm hoping that works because I really don't want to try drilling it out and chance buggering up the threads, and I don't want to pull the head if I can get away with it. I'm leery of using anything hotter than a propane torch because I'm worried of fusing the aluminum threads onto the steel plug. At least the engine is out of the car, which makes it easier to work on.
I'll post the progress as I go, but it will be slow.

 

@Jamey

Good luck ! When I got my XJS it had 3 different kinds (and ages) of spark plugs in it, and 5 out of 12 where cross threaded ! Luckily my mechanic carefully got them all out and managed to clean the threads and refit a new set of plugs so I am back to a good baseline.

Keep us updates on the progress

 

Ouch! 5 out of 12? Makes you wonder how long they'd been in there? I know they're a pain to change but geez!
I had no problem getting 11 of them out, and I'll probably attempt getting the 12th out today.

 

That stripped sparkplug was a buggar! After letting it soak for several days in PB Blaster, I heated it up with a propane torch and once more attacked it, this time with success! I didn't mess up the threads either!
Just for future reference in case anyone has a similar problem, Sears has a set of easy-outs that will fit inside the recess around the plug!

 

Good man. Keep going !

(My spark plug disaster was kind of expected. I knew my car was rough and had been neglected, and I really didn't pay a lot for it, so it was a worthwhile gamble)