Matt,

The bed with the fuel tank filler on the side and fuel tank under the bed is a 1947/48 bed. The slanted bed sides were in 47-53.

1954/55st had flat bed rails and a semi-circle embossment around the rear fenders.
1955nd to 1972 (I think) had flat top rails but no "embossment" around the rear fenders.

Where is your fuel tank?

Most of the above information is described/shown in this excellent Advance-Design Spotting Guide Tech Tip.