For some reason, the early Chevy C10 pickup trucks had an internal fuel tank. Internal as in inside the cab.
The problem with mine was that the internal tank leaked.
Add 10 gallons of gas? Prepare for seepage about 2/3 from the bottom, and gas fumes? Most unpleasant.
The solution: An external gas tank mounted behind the rear axle.
A lot of preppers do this sort of mod on older pickups when transforming them into BOVs (if you don’t know, check out www.emergency-preps.com for more info).
Usually they end up adding humongous 100 gallon tanks, though. When you’re getting out of Dodge, you always want at least 1000 miles worth of fuel on board.