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 I came from the factory with. To me stock appearing is just that and a 360 chrome air cleaner, back in the dry 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 455s air cleaners actually worked at least as well as some open elements. And In several cases better.