The fittings on the sender have nothing to do with venting. If you have the old style (70/ earlier) vent at the top of the filler neck that is all you need, and that is a GREAT system, because it places the vent point as high as you can get in the system. The only possible way that could ever overflow or siphon is if the tank is incredibly full, and you are parked on a sidehill with the filler on the low side. Even then, it would harmlessly drain out the rear frame rail until it "breaks suction."
I'm sure the sender fitting was intended for a fuel return, although the ones I've seen are too small. Return should be same size as supply, because it is LOW PRESSURE
