Correct prop size and pitch 1984 90 HP Johnson

[Texascoastalfish]

Hello all,
I finally getting very close to Sea trials on my recently acquired 1975 20' SF. I re powered down to a 90 HP 1984 Johnson. The prop is a bit beat up and motor was on a 17' tri-hull before. I know Fr.Frank and Bushwacker both have had experience with this. What recommendation for prop size and pitch ? The options are many in reading what kind of prop CAN be used. I know its a weight RPM issue. Just trying to get a starting point. 31.25 X 15 is what I heard from iBoats.
Thanks Boys.
1975 20'SF
1984 90 Johnson