Out of curiosity why do you want to add that as opposed to using the factory venting system?
In 67 for instance the vent line runs from the tank via hard lines to the top of the filler neck. At the gas station the fuel nozzle dispenses fuel below the port in the filler neck and the added fuel pushes air out to atmosphere or the vapor recovery of the gas station. Gas cap is vented so air can get in and out under normal operations.
