So, If i were impetuous enough to buy an xk8 with a bad engine (99, 4.0l, would the xj engine be a straight-up swap? I tried search, but I'm on my phone right now, and I cannot seem to find a thread that answers this. Thank you for your tolerance of a stupid newb question.

 

I view a straight engine swap as mostly a software problem. Mechanically, a Jaguar motor from another model but the same year should readily fit, except for maybe the intake/exhaust manifolds which can be used from the old motor.
Were this my problem, I would make sure that the new motor came with full wiring loom and the ECU. I would confirm that the new motor's ECU's part number is exactly the same as your old motor's. Keep in mind that with any motor swap in any car the devil is in the uncompleted/unresolved details.
I see an old XK8 with a bad motor as a "project car", which is done more for the pure fun of it as opposed to potential profit. Were this my "project" I would research a complete GM LS motor swap. It is being done with reported excellent results.