diesel tanks Ive welded on many without issues, I just did a run of 4 aluminum diesel generator tanks, modifying used tanks with diesel residue in them,no worries...but for a gas tank I try to avoid them but if absolutely necessary I would definitely purge with inert gas. I've heard of using a simple gas lawnmower engine and directing the exhaust into the tank to displace any oxygen to prevent a fire & minimise risk...for my money, a blast of argon,nitrogen or CO2 is cheap and right on my shop floor. For that matter, let the purge gas flow during welding if it's compatible with the metal of the tank/weld process..can't hurt
