See if you can spot some heavy flat beds, grain trucks etc and see how the gas filler might be mounted on them for ideas. Heres a newer GM truck, you could go to the local Chevy Dealer and probably order the recess, filler, and gas cap so it all filts properly.
Still think a radious cut above the wheels would look good. Take a pencil and some string. Tie the string around the pencil and hold one end at the center of the center cap and let out enough string to draw a radious on the bed side like this:
Since your after the old Farm Truck look some wide and long mud flaps would look good too. If you don't offend Mom some of those cool Chrome Girls too:
