The 1954 gas tanks differed from earlier years. In 1954, the "pick-up" (fuel delivery) tube and the sender are on the same assembly attached through the top of the tanks (regular cabs and panel-body cabs).
As Bill correctly pointed out above:
In 1949, the fuel tank went indoors (inside the cab in standard-cab trucks) and 3800s had a larger fuel tank than the 3100s. By 1952, the 3100-3800 cab-truck fuel tanks were the same size, 17.5 gal. I do not see any reason why a 3100 tank would not work in a 3800 (for standard cab vehicles).
