The exact-replacement gas cap for my '69 Dart that I bought from Year-One was not vented. So someone is making unvented caps. (And yes... Year-One is damn expensive)
I stand corrected then, there is an option, it will just cost you an arm and a leg. The caps you can get at the autoparts store, both locking and non locking, are vented. All the ebay remake ones are vented.
FWIW there's no functional problem having a vented cap on a '69 or earlier car.
