Anything can be done. However, you are talking about serious work. The entire front portion of the unibody is different and will have to be changed, and most sources (including Advance Adapters) also report that the firewall itself is different between '86 and '87+. I have not parked two engine=less XJs side by side to confirm the firewall change, but enough supposedly reputable sources discuss it that I'm not ready to dismiss it as an urban legend quite yet.
It sounds like to already have both vehicles, but if the goal is to repower the '86 in the long run you would find it much easier to get a Chevy 3.4L out of an early 90s Camaro and drop that in. It's the same block as the 2.8L.