The curved glass windshield and slightly different dash on '54 and '55 1st are a couple things I didn't see mentioned already. Also know that the dash is significantly different on a '54/'55 1st GMC from the Chevy.

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...