Because you're at just over recommended max rpms if your engines are Yamaha F150s (5,000-6,000) you might get away with 17P OFS4 props because their one inch larger diameter should lower your rpms by 150-300. If you try something like the Turbo Offshore OS1 which has the same diameter as the Reliance, you may still get the high rpms and wish you went up one inch in pitch.
It is a guessing game but one place to start is with Ken Reeves at Prop Gods who has overseen enough prop changes to have the best idea of where you should start. You can find him on the Band of Boaters forums where you can also search for similar set-ups and other boaters' testimonials.