Is there a formula for determining the proper gear ratio/prop pitch to horsepower?

I'm building a 383 Stroker that will produce 450-550 horsepower for a 21 ft. deep V-hull Welcraft. Right now I have a 1.65 ratio and a Mercury Enertia 21p prop. The 230 HP motor I have currently produces 45 mph at 4500 rpm. What do I need to adjust to get 65 mph from a 450-550 HP motor?