The entire tank needs to be above the fuel injector but if the viscosity of the fuel is thin, i.e. store bought diesel oil, then the lowest head need not be more than a few inches. A head of 48 inches above the pump will not overhwhelm the pump. Placing the 45 gallon drum on it's side will reduce the pressure fluctuation from full to near empty and allow you to use the existing bung holes for filling/vent and outlet.