Humm, turbo....I am thinking along those lines with my 4.6 Stroker build located here http://www.jeepstrokers.com/forum/vi...hp?f=15&t=2860
I have not had a turbo on the 258 I had when I had one but I did put in a HEI dizzy I picked up from a junk yard Chevy pickup. I also put on the 4.0 head (flows better and you need to either epoxi or weld some water ports to mate it to the 258 block) Modified the 258 intake to fit on the 4.0 head, and put on a 2100 2 bbl carb with the correct jets. She was a good running engine.
Now I am going Stroker and seeing how that will be in a FSJ.s
If you want to go for MPFI on the 4.0, check out thread; http://www.jeepforum.com/forum/f12/4...build-1114875/
For the turbo on the 4.6 I am thinnking about routing my exhaust up the dizzy side of the engine then put the turbo on. That way there is no need to make a new exhaust manifold and I can run the exahust back down the same way. I only want about 6-8 lbs of boost so we'll see how that goes.
Good luck and I will be watching as I also want to find a "junk yard" turbo set up that might work for these engines.