Another nice way to get dents out of a tank is to pressurize it with compressed air and carefully heat the dented area with the oxy-acetylene kit. With a bit of care and practice this works very well.
If you want to live long enough to have grand children, never, never, never ever pump up a tank with a gas and then wack a torch on it.
The safe way to do it is to strap the front together with some thing like 1" square tube with a 2 mm wall thickness.
Make another strap for the back using flat iron at least 1/4" thick.
Now fill the tank with water, then pump in another 500 mls ( if you can ).
This will bump up the pressure inside the tank sufficiently enough to push any dent out but if the tank splits during the process only a quick low energy squirt of 500mls of high pressure water will be let loose in your garage.
Fill it up with air and then around 200 liters of extreamly high pressure air will send the tank flying around your work shop or send schrapnel flying in all directions and oft damage the tank beyond economic repair.
Once you have the tank pressureized run your oxy torch around the inside of the dent ,just below the rim, heating only the metal that was pushed in. The heat will release stresses in the dented metal which will yield to the pressure within and "roll" out and this will happen way, way before the metal gets to a red heat and softens.
Do not play the torch on the top of the tank or you will end up with a raised welt ( much like a wart on your nose) which will be very hard to shrink back in again.