I'd leave it the way it is and choose a suitable prop. You can always change the gearing after real-world testing !
To help you with prop selection, here's a screenshot of our latest propeller calculator.
The following image is a side-by-side comparison of two engines (375HP 8.2L and a 300HP 5.7L) with .86:1 VS 1:1 gearing for each engine.
Recommended WOT range is 4200-4600 RPM for the 8.2 and 4600-5000 RPM for the 5.7 .
Theoretically, 207 shaft HP will be required to attain 50 MPH. Overpowering a hull is never a good idea.Disclaimer : Our original spreadsheet required an extensive rewrite and is still in the validation phase. It won't be released until its been thoroughly checked out !
Paul Kane Chelsea, PQ
Building the Glen-L Hot Rod : http://www.kanecustomboats.com