I tried that on a tank and all it did was deform the flat areas on the top long before it pushed out any dents and ruined the tank.

Thanks 240dkw,

the respirator is a good idea. I hadn't thought about the lead issue. If the sheet metal is that thick it might be easier to weld a tab to the outside and pull the dents out, but separating the halves gives great access for inspection, cleaning and repair.