I replaced the fuel pump on my 2008 250 and the motor itself for the pump is about 6 inches from the bottom of the tank, if the fuel cools it it would have to have at least 10 gals of fuel in it to cover the motor. the tank holds exactly 38 gallons, there is no reserve. If you would like to add a larger or second tank you are going to run into problems with the gas motor, they only make them for diesel in your year because of emissions and the fact that the fuel tank system is pressurized on gas engines on the new fords. A loose gas cap or leak will trip the cel.