My 1991 YJ came with a propane SBC 5.7L in it, but I took all that crap out and am currently putting in a TBI 5.7L out of a 1995 Suburban.
I must say that it is better than the 4.0's, and doing the swaps will cost about the same depending how you do them, but I suggest that if you do a GM V8 that you get the A&A Cradle kit for the engine mounts right off the bat. The original "fabricator" of this YJ I have did a SHITTY job, and I just don't have the time or space to cut out the crap he put in in favor of the cradle mounts.
I paid $450 for my TBI with harness and computer. If you REALLY want to do it, step up to an LS1 series engine from a GM truck or car. But you're talking some hefty $ there.
I would recommend a TBI 5.7 from a 87-93 GM truck.. The ECU's were slower back then, VS my 95, and less stuff to wire up. Or you can get a harness from Howell, or Fuel Injection Connection, or a couple of other places so that its nearly plug and play.
If interested in any of this, just PM me and I will try to help out.