Figure out the weight of your outboard and keep around that weight for the upgrade ...
Your hull should have a max horsepower rating don't exceed it too much at all , that would be measured at the business end prop or pump ...
A Yamaha 90/60 jet weighs 390#...A Yamaha 60/40 jet weighs 268#
They both have a 25" shaft length ... measured from the cavitation plate to the bottom of the transom bracket ...
http://www.onlineoutboards.com/artic...ft_length.html
Good luck with your repower and be safe ...
Barney

By the way a fishing tale posted on the main board is customary for fry ...