If you can find another 270 or 302 it will drop in easier than any of the Chevy motors, since the truck must have been modified to take the GMC already.

The Chevy six is a great motor, but the GMC is bigger and more durable. Not quite as common but not real hard to find either. And they are not expensive either.

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...