72 Demon fuel fill vent tube ideas?

If the tank is for the correct year, the vent is in the tank, NOT the sender

You tank should match the diagram I posted above.

Otherwise, I'd say yeh, weld a flush fitting in. That should be easy, say, if you can silver braze. "Mike" the fender hole so you know what you have to work with. You might be able to notch the fender hole a bit if the gasket will cover it.