I've never really done anything on a six cylinder XJ-S, but I imagine the engine bay would be far more user friendly. The V12 really does fill the engine bay even *before* you add all of the other associated bits. The V12 really needed the front of the car to be about 8" larger in every direction.
The rest of the car though would be just as much of a nightmare... there's hardly a cubic centimetre of volume anywhere that isn't packed full of wiring, relays, pipes, ducts, Bowden cables, and mysterious silver boxes with Lucas part numbers stamped on them. I've never known a car like it for just the sheer amount of stuff that's crammed into it.
Ideally... though this really is about the worst time to try to sell a car...so may just have to take it on the chin and have another bash at selling the Merc in the new year.
Oh, except for being short a parking space!