[Ken Hughes]

You would need to purge the tank with exhaust gasses from a vehicle for a couple of hours before you did anything with heat,one old way from my panel beating days was to clean the area thats damaged and solder a stick of body solder on to the damaged area let it cool and pull the stick and hopefully the dent will come out,modern body shops have dent pullers that weld a stud on and can be pulled as well.
The inside of the tank is soldered all over so that might be a problem down the track with heating it. it may be better to fill the dent or the big extreme make a new tank which i have done.