I think you need to get the static compression up to at least, an "honest" 9.5 to 1. A 273 can really work. I had a 273 in a 65 Valiant 4 dr sdn. Jahns 12.5 (nominal) pistons, Crane ss320nc cam and kit(320 adv dur, and 510 lift), Stock rockers and pushrods, LD4B manifold with 700 holley double...