I don't know if it will get the dent out or not... but it sure is dangerous as hell! It WILL however burn, and release a WHOLE LOT of energy. There is enough air in the tank to mix with the gas fumes... and well... things will get exciting real quick!
If you try it DO NOT use a match... I reapeat... DO NOT use a match. you will mangle yourself!
If your going to try it, use an electric detonator made out of a $2 5 volt relay from radioshack. Hook it up to about 100 feet of wire, and hide behind a nice big, thick tree when you set it off.
And if you turn your gas tank into a smoldering piece of scrap metal... well... what did you think was going to happen?
i'm not going to take the time to go into electrical detonator saftey... i'll just say that when your installing the thing you better make DAMN sure that it can't be activated.