Dany gets a racing stripe, or how to seal a gas cap?
Cap is vented so potential for leaks.
Not correct. The first A-bodies to use any kind of a vented cap were the '70 California models, which used a
pressure-vacuum cap (expanded to 49-state/Canada cars for '71). Prior to that, the A-body caps were not vented. The tank vent was provided separately.
OP: If you can find a hardware store with a good selection of Hillman boxes (plastic or metal sliding divided lift-lid boxes, each containing a different kind of hardware—washers, pins, nuts, screws, corks, fittings, etc) you will probably find the exactly-right-size Neoprene rubber washer. Remove the old cardboard washer from your cap. Make sure the brass tension plate behind it isn't cracked (if it is, this cap won't seal). Heat up the new washer (drop it in a pot of boiling water or boil it in a jar of water in the microwave) and stretch it over the centre hub of the cap until it's evenly in place. Done.