Well the main thing to see about is you want the
cavitation plate to be as close to just below the hull as possible.
This is the flat fin like plate that is on the lower unit.
On many sail boats you have a hard time achieving this goal.
For starters you can measure from the top of the motor mount on the boat ( assuming you have one installed already.) to the bottom of the hull.
Then measure the motor from the motor mount were it will rest on the mount to the prop shaft.
While working in a marina I saw many people who didn't have a long enough shaft on there sail boat. The prop would come out of the water when they tried to cut through a wave.
Good luck in this project.
P.S. I would not recommend sears gamefisher motors as they are unreliable and parts are hard to find.