[David R Leifheit]

Doubtful you will find anything that will work.
The truck isn't designed for a tank inside the frame rails.
*If* you were going to locate a tank between the frame and the rear fender, I am certain you could easily find one. I've had a couple trucks with extra tanks.

I am not quite sure you can do what you are suggesting anyway. Filler still on the cab, so the fill tube runs behind the seat, through a hole in the back of the cab and to the tank? That's a lot of plumbing. It would need to be a rubber tube as well to allow for movement of the cab.

What is wrong with the stock location?