Have you installed an extra vent in an early A body (I'm thinking Barracuda here)? Sounds like a good idea, but I'd like to know if there is a proven method out there. A vented gas cap could be one way, but that may allow spillage through the vent.
I bought a replacement tank off ebay , it had the usuall vent for the filler tube , and a vent sticking out above the pick up assembly (hi in the tank). I ran it around and up thru the trunk floor and up below the trunk lid , w/ 2 coils on it , and back down thru the floor in about the stock frame vent area , never has spilled or boiled over when filling , like the brand new one I bought in 1968 did .