Well if you replace the driveshaft you will have an chance to count the teeth on the gear and pinion and then you will know. If you replace the whole lower unit then yes, the ratio does matter as you can continue using your existing props. A different ratio will require different pitches to get close to the previous rpms. even then there may be a difference in Holeshot and Acceleration. Where exactly is this groove and how deep is it? A good picture(in focus, good lighting, and reasonable resolution) can be very informative