Go big. If you are committed to small block options the 388 stroker is probably best, but I'd go big block based on what you want the truck to do.

If it were me I'd go bigger than a 454.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...