It sounds like a 1-ton. All GMC 1-tons in that time were 250's, so it appears you have a 250. As far as I know that 18 inch wheels were only used on 1-ton trucks.
A firetruck on a 1-ton truck seems unusual to me. Can you post some pictures somewhere?
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...