If you crawl around under the vehicle, the existing fuel tank takes up most of the space between the transmission and the rear suspension on the driver's side of the drive shaft. The exhaust/muffler runs down the passenger's side of the drive shaft. Short of removing the spare tire and adding an aux tank, there isn't much other real estate to have a larger fuel tank. I recall that the 200 series Land Cruisers have an aux fuel tank available to mount where the spare tire goes if you move the spare to a rear bumper mounted location. Maybe someone will do something similar on the Expy.