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Actually the 69 Camaro should have a vented gas cap. Which means it should build no pressure in the gas tank anytime because its cap is always vented to atmosphere.  The newer caps that started around 1972 when they started using charcoal canisters is the pressure/vacum style cap.  It normally does not vent other then to the charcoal canister but the cap has a press and vacum release safety feature.  If the charcoal canister line got clogged or pinched and blocked off then the tank would not vent at all and the tank could swell some or shrink some when it cools down after it had been hot from a hot summer day.  So the gas cap on the sealed cars with the evap emissions systems have a vacum safety release and a press safety release in them. If the canister vent gets blocked and the car is sitting in the hot sun or driven some and sloshes the gas around it will build some press in the tank and when it gets to a certain point the cap will vent off the press by its press relief valve. And then if it sits all night after that when it cools down it will pull a vacum in the tank if the canister vent line got blocked and it could suck the tank in some but the cap also has the vacum release valve built in it.  So if the canister vent would get blocked the cap has safety press and vacum releif valve built in it which they will vent when the press or vacum gets close to being a safety problem.  The 69 Camaro gas cap should be vented all the time on a fuel system that it has which is before the evap emission systems with the charcoal canister came out.  Ron