I would start withe Mercruiser Prop calculator:
Each inch of pitch increase will drop the max rpm by approx. 200. A 21P prop would drop you down to around 4700 rpm @ WOT.
Max RPM for a 3.0L Mercruiser should still be 4400-4800.
I would be leary about a damaged prop. damage will effect the cup and pitch regardless of what is stamped on the side. SS props are much more difficult to repair an tune as compared to an aluminum. They can only be tweaked so much and have a tendency to crack.