The Hurst Olds was not a factory produced car, it was farmed out to a company you may never heard of…HURST. 421’s were put in GTOs in 1964 by Royal Pontiac, that doesn’t mean it was pontiac built.
GM did not allow any of their brands to put anything over 400 cubic inches in any intermediate platform until 1970.
Pontiac had many 1sts…I actually have a link to PMD’s engineering firsts in my home PC if you really need it, but this details Pontiac’s earlier years in racing…