I've been helping a guy get his '73 D100 up and running, but the gas tank behind the seat was removed at one point in the trucks life because the previous owner put a huge camper on top of it, and removed the in-cab tank. The camper is gone, now and I'm just trying to find a tank to replace the factory tank, but keep coming up empty as regard these behind-seat tanks. Why do they seem to be unobtainium? I can't seem to find one, anywhere!

Matthew