Topgun,
Tell us a little more about your boat... What kind of hull is it? As GoFast said, it's not a racer so I'm assuming it's a laker... Crackers are direct drive so you need to spin 'em to go fast. We run the race boats at 8,000 - 9,000 rpm, but we run 16 to 1 compression with 118 octane. We are limited to 314 CI., but the fast guys make over 600hp.
If your going to turn 6400 max I would recommend an 11" 15 pitch 3 blade. The neat thing about direct drive is that it will accelerate very hard. You will probably only get about 65mph with the motor and rpm you plan to run but it will get there in a hurry!
One last question... Why run water-injected headers?
Are there noise restrictions in your area? Are the headers through the side style or above the deck? If they are above the deck it won't be fun to ride in:frown:
S&S