Like Scott Said and not trying to be smart...but need more info.
Current wot rpm when normally loaded and trimmed efficiently
Current propeller pitch
Current gear ratio.
If it makes a difference...a guess (which I shouldn't) would be that it's a 1.81 ratio and probably a 19-21" prop would be the norm.
Likely Jiimbo, All dodge, searider, Bruceb or similar experts will be along shortly to comment...worth listening to, if they do. But they might not entertain it until the above info is stated.
All the best.
