AIR TIME

you can call brett at bblades he has a test program=cheap money. the 2 props I would try is the rev 4 by merc in a 23p, its a small diam than the b1 props and works well with deep drive boats as yours. the other is the mirage 23p, I have a turbo 24 3 blade that worked great on my stock 502. I'd ask 250 for it. it uses the same rpm of a 23 m but was as fast as my 25m. another think that 5 blade was made for sking. also check your plugs/ water/fuel separater needs to be changed each season. if I can help let me know. I did 5 motor changes over 15 yrs with my ol24, went from 60 to 80 plus over time. ps change the fuel filters for sure old gas is bad gas these days.

WHIPLASH

My 89 251 has a 454 mag with a Holley carb and GLM exhaust manifolds with Fastglass stainless risers. I mention these because it has a little more power than a standard 454 mag so I go a little faster than most 251's with a 454 mag.
The best prop I use is a Mirage plus 23p tuned. I get 69mph out of it at 4900 rpm. I was told to use the Plus over the regular Mirage as it gives more bow lift and I've found that to be true.
I also use a Rev 4 21p that will do 66 at 5000rpm. I have a 23p Rev 4 that only does 4400 rpm but your 502 has more power and might handle the 23p Rev 4. If you think that may be a prop for you I will be selling it.
I also use a Bravo one 22p 4 blade that is great for rough water and cruising but is only 65mph at 47-4800rpm.
I hope this helps and doesn't make it more confusing.