If some of this makes no sence, don't worry about it. Just answer what you can.
What RPM does it spin at Wide Open Throttle, right now, as it is?
What are the propeller dimensions? (diameter x pitch)
How many blades does the propeller have?
What v-drive is in the boat?
What gears are in the v-drive?
Is there a transmission? (fwd/nut/rev) If so, what type?
What cam is in it? Brand & Part# or cam card specs are good.
Cam... Solid/Hydraulic/Flat tappet/Roller?
Intake? Tunnel ram, single plane, dual plane, supercharger...
What carburetor(s)... or fuel injection
What cylinder heads? Aftermarket or GM casting# please.
Heads... stock or ported?
Cylinder heads Iron or Aluminum?
2 or 4 bolt block?
Cast or forged crankshaft?
Connecting rods?.. Stock GM or aftermarket... brand/type?
Where do you get the figure of 500hp?
Was it run and tuned on a dyno?
Is there a dyno sheet you can post?
The more info we can get, the more accurate we can be in aiming you down the correct path. And you'll spend less time and money getting where you want to be.