Ford makes one fuel tank, the same tank is install whether it is gas or diesel. For what ever reason the gas system requires an expansion area above the surface of the gas, probably emission controls, the diesel does not require the expansion space. To have this space above the surface of the gas, the fuel tank vent protrudes 5" to 6" inches into the tank. When you "Harpoon" the tank, the tank is removed and opened, using a PVC pipe cutter, the vent and fill pipes are cut as close as possible to the top of the tank and this allows for extra capacity and quick filling of the fuel tanks, about 33 USG in the 28 USG tank, and about 42 USG in the 38 USG tank.