Doesn't anyone solder anymore? Use two big coppers, heated by a torch a few
steps away from the tank and above it, then there wil be no source of
ignition. Just keep swapping coppers as they cool.
http://tinyurl.com/ybh4d86

Since you are planning on TIG welding, we know you have Argon. As it is
heavier than air, plug the other holes and fill it from the top, no oxygen,
no fire.


I have heard of tanks filled with water still going off, the fumes will
still be in any air space, so the preceding two methods are safer.
--
Stupendous Man,
Defender of Freedom, Advocate of Liberty