May be embarrassing myself here, but from a 40+ y/o memory, it wasn't the gas tank per se. I think it was a misplaced bolt/screw from the driver's seat that was puncturing the gas tank during rear end collisions.
But don't quote me on that...
Front tank mounts were suspended from the frame, allowing them to break off and the tank to shift forward (into that bolt).
Gasoline would spill onto the heat of the exhaust system and fire would result...under the front driver seat.
The initial design for the Pinto fuel tank was for it to be self sealing but it was canned as a cost reduction.