[Kahuna]

SC,
I have a 20' seafari myself with I/O, but two weeks ago I ran a clean 75 20' CC, with a very new 130 2-stroke yamaha. What a ROCKET!! full fuel, two people + dog, if you weren't holding on, you were going out the back. Based on the transome height, I would not put any more weight on the aft end of that boat even if the transom was 4-5" higher. I can't imagine 15 less hp would be a big production. The difference you will have between a 1977 outboard and a new one is going to be substantial. I do know somebody who is redoing a boat just like mine, raising the transom 5" adn plans on putting 115 2-stoke on it. 6 cylinders is too much weight, in my own opinion of course...