Isn't RPM the Key to MPH?
 

It seems like you could calculate how much more RPM you would have to turn the engine to achieve your objective. (Engine RPM X OD Ratio X Prop Pitch subtract for Prop Slip) RPM @ 45 MPH could be 4800 rpm, RPM @ 55 MPH could be 6000 RPM. Still doing 45mph at 4800. I am more than 3/4 of the way through building Twin 427 Stroked small blocks that dyno tested @ 450hp and 500ft.lbs NA with 215 heads 9.5 to 1 and 276 cams. Peak Horsepower was made at 5300 RPM and peak torque was at 3900. These engines with the addition of twin superchargers should easily do 600/600. I am building torque monsters so I can change the outdrive gear ratio and ultimately lower my engine RPM..... Without significant gear ratio change 300hp at 5000rpm is the same MPH as 600hp at 5000RPM.

I used to build a few SBC back in the day, most of the 350+hp 350's would not be suitable for marine use, but that was hot rodding on a budget lol. I would think you could get there with "new" technology in heads and cam combo. Before I would spend the money on your heads, I would consider some out of the box ready to bolt on aluminum heads. Those and a good marine grind cam, and intake/carb combo you should be real close and not breaking the bank. If you are even considering removing the engine for low end work of any kind, I would highly recommend considering a used B1 set up with a complete ready to drop in big block. I see them all over the market place for under 10g. Your alpha set up should bring close to 1/2 that, which makes the B1 set up very affordable.

I have a Mirage 23 footer with Volvo 350 and duoprop drive. I top out at 58 mph on gps with 2 people and no gear on board. I am very happy.

350 hp? big block right?

No It’s a Volvo 350 small block fuel injected and duoprop. It is supposed to be 300 hp but I have read it’s more like 320. The seller told me 55 but when we gps it’s 58. I have talked to guys with the lower horsepower 454’s in the same boat and they get about 62 with single prop.