GMC should be stamped in the tailgate. A 1940-1946 bed will be 48 1/2 inches wide. Earlier beds were narrower, later beds were wider. A 1940-1946 gmc would have a corrigated steel floor, not unlike modern pickups. Chevies of the same era had wooden bed floors.
I can't tell if your fenders are original or not from the 2 photos you had. They are a bit banged up for sure. If you could post more pics I'll let you know.
John
