UPDATE:
I changed the heads on my boat (SBC) from 200CC runners to 170CC runners, advanced the cam 4 degrees, what a difference! I gained 3-4 mph and alot more torque, I have a hard time spinning a 21P Mirage still but I have a labbed 20P Mirage and it seems to be perfect so far, 4600 rpm at 52mph best so far. with the 19P rapture I get 4800rpm and 47-48 mph. with a 21P Mirage I get 4600 rpm at 50 mph best.

I need to measure diameter, would a more pitch smaller diameter help? am I probably at my limits as far as speed? or can I find 1-2 MPH somewhere??

HELP!!!