Welding on a used gas tank can be dicey.
An old friend, Art Morrison, told me a story about a time he was welding a fuel tank that he had put water in thinking it would safe that way, long story short, it blew apart with in seconds of starting his welding.
I have welded several tanks and feel the safest method is to purge them with argon before and during the welding process.
I have also welded motorcycle tanks, by running car exhaust in them for a period before welding, it worked slick, an old Harley mechanic showed me that one.

I hate to see a fellow car guy get hurt, just thought I would toss that information out there.