The '72 and '73 models have the filler located on the side of the cab and the fill pipe enters at the bottom edge of the tank. Later models have the filler on the side of the box and the tube enters the tank at the top. The only way to use a later tank is to also use a later box or to modify the '73 box sheet metal, and that leaves you with the decision of what to do with the unused filler on the cab.