gas tank sodering

Just take it to a radiator shop and have it "boiled out" and repaired. It's not that expensive, it'll clean all the rust and crud out of the tank, and you'll have the piece of mind knowing that it was done right. Not only that, any reputable shop will pressure test the results, so you'll know it wont leak too.

For the OP, exhaust fumes contain A LOT more than just C02, including lots of unburned hydrocarbons (fuel!) and CO which can still ignite. Hooking a gas tank to a tailpipe and welding on it is a great way to end up in the ER.

I can MIG, TIG, torch weld and braze just about anything with the machines I have in my shop. And the one thing I don't mess with is fuel tanks- unless they just came back from the radiator shop. Not worth the hassle or the risk.