Ok, I am running the 496 mag, I am spinning a Bravo 1-24 pitch, I was running about 4700 RPM.
Mine originally had a 24P on it. Mind you, I am running 30 feet and an HO, not standard mag. I was seeing around 48-4900. Took a little while for the nose to roll over on plane.
I got a 22P (smallest Merc Bravo-1 4-blade) and am right around 5100, a lot nicer out of the hole - boat rolls right over on alot less throttle. My speed picked up from 61-62 to 64-65.
I still have my 24P in case I bring the HP up to pull it. But, I run the 22 for all around fun. Oh, and the motor is stock but pulled the turbulators.
I would borrow a 22P and see how you like it on the boat. If you end up doing that and like it, hang on to your labbed prop. You may need it some day.
EDIT: BTW, what boat is that?