So I was thinking about this some more and said **** it and went and took some measure ments.
So between the frame rails is just over 36".
From the rear bumper to the next cross brace going forward that has body mounts on it that I wouldn't want to spend the time modifying is a little over 30".
And from the lowest point on the body to the bottom of the factory rear bumper cover is 10".
So just using those measurements you have a volume of over 46 gallons.
Excuse my dirty truck but you can see there is quite a bit of room there. You would need to remove the 2 vertical support beams that connect with the plate that goes across the top of the frame. Then you can just add another channel support near the rear bumper.
And the tank could be taller than 10", closer to the middle of the truck since the body raises up a little bit and it can hang lower since you wouldn't have to worry about it getting hit on anything.