pretty sure a 4.0 will bolt on as far as block to bellhousing pattern, but not sure which flex plate you'd have to use for it to work... someone who's more familiar can chime in on that.
Other than that, I think the 258 pattern is also same as the small block AMC V8's, but that rules out FI as far as I know... don't think the 304 or 360 ever came available with FI.
I THINK that's it for what most would consider "bolt-on"... next you'd be moving on to Advance Adapters or possibly Novak for an engine - trans adapter plate... then you could go with a Chevy 4.3, 5.0 or 5.7
another option if you're really looking to get rid of the 4.2... if you have documentation on your puchase of the new trans, sell off the trans and use that toward getting an engine/tranny/tc combo... if you want to keep with an auto trans, you could easily end up with an OD trans, gaining the additional benefits. Then you can go with a 4.0/AW4 from a Cherokee, a GM 4.3, 5.0 or 5.7 with 700r4 (or 4L60)
sure swaps aren't simple... but usually goes much smoother if you're keeping the whole powertrain together
[FONT="Comic Sans MS"][COLOR="Blue"]Sean B. - Schenectady, NY - 95 YJ, 350/700r4/NP241, JB Shorty SYE, 8.8 rear, 2" BDS, 1" homebrew shackles, Daystar 1" BL, TJ flares, 35x12.50 General Grabber AT2 on 15x10 Bart D-Truckers, Polyshield polycarbonate windshield, HD over/under crossover steering
[/COLOR] [COLOR="Red"]swap/build thread: [url]http://www.jeepforum.com/forum/f22/after-almost-4-years-finally-my-own-build-swap-thread-1542284/[/url][/COLOR][/FONT]