That's a 15 pitch prop...very surprisingly small pitch, given your current rpms. I assume you have a 1.98 gear ratio since that's pretty much what all the 3.0 Mercruisers were/are...can you confirm this figure? Maybe it's been swapped out.
I'm curious what your top speed is when you are at 4,000 rpm? According to prop calculator, this boat is challenged to get much past 25 mph? Is that true, or is the boat much faster than that? If it's faster than 30 or 35, I will assume the outdrive is not original, and the gear ratio has changed...
Something just seems off, and I'm not convinced it's the prop alone.
When was the last time this motor has been treated to a fully comprehensive tune-up? Compression check, new spark plugs AND wires, distributor cap & rotor, points (if equipped), set base timing, check advance timing, throttle cable adjustment, fuel filter clean/replace, etc. etc....
I highly suggest you consider doing all of above before trying to reprop the boat. IMO you seem to be down on horsepower for some reason....but maybe not if your speed is higher than I think it is.
and thanks in advance.