Those motor's are designed to run a 16in dia prop. That is what bite's the water hard to give you the holeshot.
Going to a 16in dia is going to reducer your RPM with the same pitch as the 15.5dia.
So something in 3 X 16 X 16. 17in pitch is the lowest pitch in stainless they list.
When you get it have it checked at a good prop shop that has computer redout's. I bet it will be lower than 17in pitch.
I have 2 -14 X18 prop's for my DF-140's. They checked out 16 3/4in pitch and 17in pitch. I hade them reworked to 18.5 pitch.
http://store.brownspoint.com/suzukiprops15.asp