People used to take XJ6's (I'm pretty sure this is correct) and put 5.7 Chevy engines in them all of the time. There was an issue with the aluminum heads warping and costing too much to repair, so dropping a common 350cu in was a good alternative. It would be a good move for a XK, they had reliability issues from what I've read, a LS6 would be a good solution.
I think about what I'd love to have and what engines would be better all of the time, I tend to always go back to mid-engine Porsches with small Subaru boxer transplants as a dream.