Radiator shops don't normally braze or weld them, they don't even bother to get the fumes out. They use a hot soldering copper and solder them. Since there is no spark or flame there is no explosion hazard. On my 73 T/A I had a leak at the soldered joint where the neck joined the tank and I fixed it with a copper and re-soldered the joint.
If you want to weld one all that's needed is to hook the exhaust pipe of a running car to the tank taking the oxygen out of the tank.
Gas tank repair with epoxy:
http://www.gastankpatch.com/