Donor year doesn't mean squat when you're dumping into an 86.
Shoot for 91-95.
Regardless of year donor, you're going to need to modify the radiator supports, and possibly pound on the firewall a bit.
This is on top of needing to swap ALL wiring, dash and engine bay, along with all the cooling stuff, vacuum, etc.
Best way is to find yourself an XJ that was wrecked either from behind, or not too bad up front.