the jeep 4.0 is a great motor but would be an extremely tight squeeze being that its an inline 6.
the chevy 4.3L v6 is a great option to go with. good power, large aftermarket, adapters are already made to bolt it to a toyota tranny (or take a gm tranny and mount it to a marlin crawler crawlbox). good fuel economy from a v6 as well.
2.7L inline 4 from toyota is another great choice. good power and fuel economy.
i wouldnt go with a v8 gasser, too much for one of these little trucks really.
or you could go with a diesel. motors that fit easily are:
toyota 1kzt
toyota 1kzte (check out the willy mammoth on this forum for how to do that swap)
nissan sd22, 2.2L that is great on fuel but has no power
nissan sd25T, turbo charged 2.5L, can make decent power
tighter fit diesels but more powerfull:
nissan sd33T, 3.3L turboed that came in the international scout, inline 6 that can make decent power
cummins 4bt, 3.9L inline 4 turbo that came in the box trucks (UPS, Fed Ex, etc). 240-265 ft/lbs of torque stock depending on the year. this motor is HEAVY for these little trucks (about 800-850 pounds if i remember right, a chevy 6.2L or 6.5L diesel is about that) and can easily make too much power for these little trucks to handle (frame snapping kinda torque). stock form for this motor would be wonderfull in these trucks, but requires stronger front leaf springs, and some frame reinforcement would be a very good idea. excellent fuel economy and an incredibly long life span though. tight fit vertically, body lift might be needed.
