Geez, I'd be scared to death to weld on a Model-A gas tank!
Ohh wait, I do weld on Model-A gas tanks.
My suggestion is use your instincts as your guide. Then common sense.
I will tell you from personal experience that a drained gas tank (-with fumes only present) that has the filler neck, gas gauge opening, and fuel outlet bung all open to atmosphere does little more that go 'woof' for a brief second when introduced to fire.
FWIW, I have tried to intentionally see what it takes to make one explode. Using starting fluid to spray a fire trail to the fuel fill opening, -and with a drained tank I had a bigger fire with the starting fluid than the inside of the tank. As someone mentioned above, if you are concerned, fill the tank with water and then weld all you wish.