4.3 will fit BEAUTIFULLY!, ever see a tracker with the engine pulled? theres a ton of room.
350 v8 would be nice but pretty short front to back, so i could see where the 4.3 works out better. stock tranny might work, if you had an adapter, but for how long? you can probably use s10 5speed and transfer case or a 700r4 trans.. and those 4.3's can really fly!,
Easiest bet would be to order up a edlebrock carb and 4.3 intake, headers and a nice lopey cam...
Stupidest move i ever made was NOT buying a 350v8 powered samurai a few years back.. that thing would like the front wheels off the ground a foot when it got 2nd gear!, but it was a little tooo insane i think, and you had to be a gymnast to climb into the drivers seat with all the roll bars.. who really wants to doo all that when your tired int he morning stopping at starbucks for a coffee on the way to work? heh..