[BRENT in 10-uh-C]

My experience with repairing these tanks is that there is a reinforcement flange on the inside that the rivets are bucked against. The tank metal usually is cracked (- generally caused by removing/tightening a fuel valve) and the correct way to repair this is to weld up the holes & cracks in the tank metal and redrill. Then re-rivet the flange & buck back in. While you can access it by cutting a hole in the backside, I would cut the entire tank open and do a complete restoration on the tank.