I have a 69, but my tank has a vent tube welded in the neck of the tank. On this was a hose that vents thru my rear tail panel in between the bumper and is not visible to the casual viewer.

There are also vented caps that you can buy. I would get a vented cap and see if your tank is vented. Again, not sure about 67's but if not vent in the tank itself you most likely need a vented cap.