Since it sounds like you hauled it home in a box, here's my opinion:
I would say that dropping a Small Block (305 or even a 350) in would work fine, and be fairly easy, provided that it came with a V8. You can go with a variety of transmissions (e.g. SM420, T5, et cetera), but I'd stick with a Chevy rear from that time period. It'd just be easier to put in, and if you grab a 1/2 ton axle, you'll have the right sized brakes and everything!
If it came with an I6, then I'd go with a 235 or a 261, just because it'd be an easy swap and I'm lazy. You can use a variety of transmissions on these engines, too.
Since there IS no brake system or front axle, you'd need to find those, too. Again, I'd stick with a Chevy Setup from the same time period for ease of installation. If you're worried about stopping power, the drums work fine so long as you're not hotdogging -- which it sounds like you won't be. There are disc kits available, but if the donor truck doesn't have a hydrovac (or other brake booster) then it will be harder to brake with discs than with the original drums.
