Bond-o already mentioned it... but it needs to be emphasized. If you are building this motor from scratch one of the more important decisions will be the cam. What works for power in a car is different than what you need in a boat. It needs to be simple without a lot of overlap. Your basic stock cam that came in a half ton truck is basically what you want. Or if you are ordering a cam - just be sure you do the research for one that builds power (torque) on the low end of the rpm scale. Your outdrive will not "allow" anything over 5,000 rpm.
Ayuh,..... Back in the day, if yer camin' yer pickup truck, ya went 'n bought an RV Cam,....
When ya got the cam, the tag that said RV, said Marine on the other side,....
If yer buildin' from scratch, 'n lookin' for the "Plan",...
Build yer HP with compression, 'n Vortec fast burn heads, 'n matchin' pistons,....
You'll need a dished, or inverted dome piston that mirrors the head,...
Shoot for right at, or just under 10:1,.....
A Stroker crank will Really up the ante,....