If I remember well your boat is probably an 18foot bubbledeck model from the seventies{I'll try to go trough my old mags to find some info}.The hull makes no difference on your RPM but the impeller size does.The 383 wouldn't turn 6000 rpm with an A impeller.First you need a good tachometer to take the guesswork out.To achieve 6000 you would have to turn the impeller down to a b size at least may be even a c .Around 5400-5600 rpm would be my preference to find.I remember the 340 engine in my 72 challenger was happiest at around 5000.Boats need a lot of torque to get on plane and your 383's torque lower than the BB chevys and Fords.My friend's 68 Charger was flying w/383 but that car was very light.Check your hull's buttom for hooks at the end part.