I haven’t raced in BSA yet but plan to in 2022 so my opinion might not carry much weight. That said, I think each entrant should have a stock air cleaner that it came from the factory with. To me stock appearing is just that and a 360 chrome air cleaner, back in the day usually meant that the motor was “probably “ modified in other ways. By the way, some tests have shown that the stock base on 68-72 455 air cleaners with an inverted top or a taller filter element actually worked at least as well as some open elements. And In several cases better.