With that old truck being - an old truck - are there others around you could get a cheap (good) motor from a bashed truck ? Might make the job easier !
If you want to put a 5.0 302w in it that could also work , probably a lot of bits needs changing though - header/manifolds , fuel lines , wiring , radiator tubes / links and
oil pan clearances / design .
Make yourself a checklist and go through each system and verify the solution