What speed should I be seeing in a 1978 30' Scarab
 

I just picked up a 1978 Scarab 300, 330 HP BBC old rear exit log manifolds with thru hull exhaust, TRS drives spinning pout with 24" 4 blade props ( they look like Hydromotives but don't have a name on them), Merc Trans. From what I have read the RPM's should be 4500. What kind of speeds should I get out of this hull with the 330's?

Mid-50's on GPS. Want to go faster? Install a (non GPS) Checkmate speedometer.

That is about what I get. Im seeing 55 on gps