In 1958, the 21 foot Century Coronado had four V-8s available. There was a Gray Marine V-8 at 225 HP. There were two Chrysler V-8 options, one at 225HP and another at 275 HP.
There also was a Cadilac V-8 at 275 HP and last, there was a Ford Interceptor V-8 at 215 HP.
In 1959, the Coronado got a totally new hull and the max HP was upped to 325.

NB
(Ref: Speltz;
Real Runaboats II)
PS: The biggest V-8 I can remember in 1958 was the Chevy 348 ci (Truck engine) that was put in some Chevy Impalas. I'll stick my neck out and guess that 427s didn't come out until the mid sixties. Your boat may have been repowered in the sixties.