340 or 408?

If you want to guesstimate peak power rpm for a given cid at around a decent 1.25 lbs-ft per cid.
Obviously if less efficient more rpm, higher efficiency lower rpms to make 500 hp.

500 hp
2,626,000 / (cid x 1.13 ) = peak hp rpm

(Hp x 5252) / (cid x 1.13) = peak hp rpm