Two other things to add to Jon's info above:
1) Be sure you have a vented gas cap if you do decide to route a return line to the filler neck, otherwise you'll have no way for the tank to vent.
2) I tried to use the filler neck vent for a return for my fuel injection. The fuel returning and spraying into the filler neck is fine until you decide to remove the cap while the truck is running. It will give you a nice shower of fuel! I made a 90 degree fitting to turn down into the filler neck and direct the fuel downward. With the F.I. there is some considerable volume. With a mechanical pump it is much less but still something to consider.
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