Slant six or v8 swap for use in a truck

if it's your daily, don't mess with it.

but let's play devil's advocate here a moment. going to V8 will require a whole host of accoutrements for the extra two cylinders. for starters, you'll need a V8 bell housing and all the peripheral bits- Z-bar, fork, possibly a different flywheel & clutch. you'll need to add and modify your exhaust. motor mounts. a bit of wiring. sorting out of the radiator inlet/outlet in relation to the corresponding bits on the block.

and that's all just to get it in there.

the rear end? i dunno, i'd probably try and find one at a 'yard and just stab it in. though if it's anything different that what's en there, expect to pay to have the driveshaft shortened and other faffing about: e-brake cables, brake line adapter fittings/lines, possibly u-bolts and shock plates. or you could see about locating some gears, maybe used and swap those in, and if you're not setting them up yourself that means pulling the rearend out and taking it to somebody to pay them to do it.

my guess is that you'd probably spend about 2K swapping over-- not including the cost of the motor.