Vapor return or liquid return line?
Use the tube at the top for your return. It originally was a vent via the charcoal cannister, it doesn't work well as an atmospheric vent since it is in the main body of the tank and can be below fuel level when the tank is full; so use it as the return. From your original post I'm assuming (maybe I shouldn't...) that the emissions and evap systems have been axed. If that is the case, you will need to vent the tank another way- either by adding an early-style vent tube to the filler neck as suggested by others above, or by using a vented filler cap. *I believe* that a Chevy (I know, I know...) pickup gas cap from the mid '70s is vented and fits a Mopar filler tube, but due to filler variations over the years I can't say for sure it fits the '73 filler neck. Maybe someone else can confirm this particular combination one way or another. I know the Chevy caps were even being sold by some vendors as a replacement for certain Mopar caps, but I don't recall which years.
Use the tube at the top for your return. It originally was a vent via the charcoal cannister, it doesn't work well as an atmospheric vent since it is in the main body of the tank and can be below fuel level when the tank is full; so use it as the return. From your original post I'm assuming (maybe I shouldn't...) that the emissions and evap systems have been axed. If that is the case, you will need to vent the tank another way- either by adding an early-style vent tube to the filler neck as suggested by others above, or by using a vented filler cap. *I believe* that a Chevy (I know, I know...) pickup gas cap from the mid '70s is vented and fits a Mopar filler tube, but due to filler variations over the years I can't say for sure it fits the '73 filler neck. Maybe someone else can confirm this particular combination one way or another. I know the Chevy caps were even being sold by some vendors as a replacement for certain Mopar caps, but I don't recall which years.