67 firebirds had a vented tank cap, not non-vented didn't know they had a fuel return line on 67's. It sounds like your tank is building up pressure, if you can't find a cap, or clean out the check valve in the cap I would leave it loose or it'll blow out the hose that connects to the tank to filler neck. I went looking for a cap earlier They don't seem to make one at the moment. NOS would be the way to go.
