I have a 79 Renken 21' cuddy, 165 hp. 250 cu.in. six, with mercruiser I outdrive. This outdrive has 1:68-1 gear ratio, but unfortunately water seeped into the top portion of the gearcase and ruined the gears. I have a very good M-I outdrive to replace it with, except it came from a boat that had a 4 cyl. and the gear ratio for this outdrive is 1:98-1. A friend told me not to change the gear ratio in the new outdrive, but purchase another prop that would offset the change in rpm to get the same performance. It currently has a MW 15-1/2 dia, 19 pitch 3 blade prop. The boat tops out at approx. 35 mph@ 4000 rpm. Any help would be appreciated.