From my own research, there are front crossmembers w/ mounts setup available. And with some minor fab skills a rear crossmember can be worked up. If you are going fuel injected than there will be quite a task ahead of you in swapping/splicing the wiring harnesses. Carbed and it would be easier on the wiring, but you would have to set up the fuel delivery correctly for a carb (stock fuel injection setup won't work). Routing the headers can be quite difficult on an XJ, although I believe there is a couple companies making SBC swap headers for the XJ out there. Cooling is a tough spot too, since the stock XJ cooling system is totally inadequate for a SBC. This will require some careful thought, and likely the use of an angle grinder to cut out some material to get it to fit nice.

Picking a tranny/transfer case seems to be tough for some, based on what will fit, and what kind of drop the transfer case has (driver drop, or passenger). If you are fortunate enough to have an AX15 manual, then a bellhousing adapter is all you need to have a drivetrain setup that works with the V8.Then there are the driveshafts they would likely need to be custom made to fit. And your axles probably won't hold up to V8 power for long, especially with a Dana 35. So an upgrade to at least D44s or a D44/Ford 9" setup would work.