Not that long ago, you could find those big soldering irons at garage sales, swap meets, just about everywhere - but I guess the supply has about dried up since "nobody uses that type of thing anymore." I had a pretty big one that I used from time to time, but the element finally gave up the ghost.
You can also use a propane/map torch if you don't have access to an iron. Or even silver solder if you want a stronger bond. BUT for fuel tanks, you have to get all the gas vapor out first. Pretty easy for small gt size tanks - get all the gas out and fill/drain with water a few times to get all the fumes out of the tank. But a non-flame option is probably safer!!!