Hey everyone. I recently picked up this 83 charger bubble deck with a 403 olds and berkeley 12JE pump. I rebuilt the carb, replaced the fuel pump, and took it to Silverwood Lake to test it out. The boat overall ran pretty good but it would only get to 4000 RPMs and my gps speedo clocked it 42 MPH top speed. is that normal? I feel like I should be getting more RPMs and the guy I got it from said it would do somewhere around 60 MPH so I'm a little disappointed to see 40. Should I expect more out of this setup? If so how do I get it? Any help is appreciated

just a little more info: the carb is a holley 600cfm double pumper on a edlebrock performer intake. hardin marine thru the transom exhaust. And the engine has some 3A heads I think are from a 350 olds