the early 70s vented caps only vent one way, in. they dont vent out. the rubber diaphragm lets air in when needed but prevents gas fumes or raw fuel from going out.
a charcoal canister will prevent gas smell but the small amount of vapors it catches wont do anything for saving money on fuel.
the issue with stalling at the post office is also most likely not vapor lock... especially in december in PA, unless you have fuel lines touching exhaust components or something causing excessive heat.
i have a 1970 non vented tank in my 72 firebird & eliminated the evap canister, also have a 78 & 81 i eliminated the charcoal canisters, i keep 2 of the cars parked in a 2 car garage & never notice any strong gas smell so for me the canisters are not a concern nor will they save me any money on gas.
i use a tanks inc one way vent for the 72 that is safer than just running a piece of hose for a vent, keeps the tank vented to atmosphere & prevents fuel from spilling out in an accident that would flip the car on its side or roll over.
https://www.tanksinc.com/index.cfm/p...prod/prd98.htm