Just a 350 small block would be a good engine, easy to get one and put in too.
If it were me though, I'd build a 301 for it. Far from being the fastest thing around, but you don't ever see them, and it's a cool old school engine.
It kinda depend on what direction the whole truck is going in though. For example, is it gonna look like a bone stock, nicely restored 55? Or a hot rod truck? Lowered weekend cruiser? To me, what kind of engine goes in it kinda depends on how the truck is built. JMO though