Re: !!!need some serious info on proper prop size!!!
You replaced a 1.5 ratio gearbox with a 2.0 gearbox, which is numerically 33% higher. Boat probably originally had a prop in the neighborhood of 21" pitch . To make up for the 33% numerically higher outdrive, you'll need 33% more prop pitch to get your engine RPM back in the ballpark. Means you'll need somewhere around a 28" pitch prop. Biggest Alpha prop I'm aware of happens to be 28" pitch made for the couple of years they had the 454 in front of an Alpha. They're relatively rare, probably hard to find used and will cost you $500-$600 new. Then it probably won't work worth a darn on your boat due to the different slip ratios and prop efficiencies of the steeper pitch.
You could gamble your money on the high pitch prop, but your best bet would be to sell your drive and buy one with the correct gear ratio to get your prop pitch where it should be.
