I've never heard if "Welding" the sending unit to the tank. Every one I've seen is held in place with screws, which can be removed to service the sender if needed. Why would you want to weld it on?
Don't think I would want to be in the same neighborhood when someone were to attempt to weld a sending unit to a gas tank. All (some kind of a standard I think) sending units are attached to a tank with 5 ss screws. The screw holes are not evenly set apart so that the sender cannot be mounted incorrectly.
