[Aus69c20]

I think the best way to get a return into the stock tank is to modify the fuel sender or fit a 71-72 sender which has an extra 5/16 line for a return. By doing it this way you don’t have to touch the tank.
Also try to return the fuel close the bottom of the tank. That way you don’t aerate the fuel and you won’t hear the fuel returning to the tank.