I had a couple of dents taken out of a tank from one of those paint less dent repair places. Was a neat process. No glue, just took off the gas cap and worked them out from the inside using a couple of different curved tools with rounded ends.
granted, the dents were smaller than the OP, but when he was done, you could not tell there were ever dents there.
curious to know what happened though. Something fall on the tank?