nytrdr24
+1y
ford 302 would be the way to go as far as the engine, you won't have any clearance issues with the distributor & firewall....
as far as the tranny question goes, that is mostly preferencial, if the truck is a 5 speed now, it would be easier to do the manual tranny, like wise if it is an auto, it would probably be easier to do an auto tranny....
either way, you will need to source some axles for the front & rear, there is no way that I'd build one with a v-8 & expect to keep the weak irs in the front, or the small rear axle as the torque from a v8 would make minced meat from either one. you will also run into fabbing up mounts for the transfer case / tranny combo, and have some issues to deal with the driveshafts.
all in all, it would be a very ambitious project, but sweet once it was done!