Depends on where it is. If it is accessible from the gas cap, you can make a bent 1/4" or 3/8" rod with a steel ball on the end and work it from the inside out. If it is not accessible from the cap, you can make yourself a little slide hammer and tack weld to the outside of the dent and pull it out. BE SURE TO DE-GAS THE TANK BEFORE WELDING ON IT.
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This part is VERY important!!!!