The return line coming from the pump isn't a vent for the tank. It connects to the sending unit assembly on the top of the gas tank. If you used the same sending unit you had before the tank change you should still have the tube to connect the return line to. You can buy a pump for a 350 car(w/o ac). It doesn't have the return line but it may not be big enough to feed your 400. Depends on how you drive.