12 gauge? Wow. You shouldn't burn through that, SB and it might even shield you from small arms fire, just in case you get into a furball confrontation with the bad guys.
Take your time and tack weld then connect the tacks. You should be good. If in doubt about the tank's ability to not leak let me know. I have a good source for tank sealant that works and is impervious to ethanol gasloine.