I'm looking at a 89 Fountain (non step) w/ 500 h.p. 's w/ # 3 drives. It has 27" 3 blade cleavers and cruise speed is 50mph @3200-3400 rpm and 64-65 mph @4600rpm. (this is w.o.t.) My quest. is w/o increasing hp. what prop size will result in best rpm.increase at w.o.t. 3or 4 blade and what would top speed would I get from the change?

You need to increase your RPM by 600, so approximately 3 inches of pitch and similar diameter to get you in the ballpark with a 3B propeller.

Julie