Behind the seat tank in a square body.

[halfgassed]

Years back I had a 86 chevy that I wheeled and did not want the saddle tanks hanging, and the way I bobbed the truck there was no room for a between the frame tank or fuel cell. I used a behind the seat tank from a 69 GMC and mounted it inside the box between the box sides up against the front of the box. The filler neck was perfect height to be just above the box side.
If you are going to put a flat bed with a roll bar on it why not mount it between the front bars of the roll bar?