Hi Alexander. The 750 makes a great float plane! I believe another engine you may like is the Viking 140 hp which would be plenty of power. I removed the leading edge slats from the wings as water TOS and landings have plenty of length. My useful wS just over 400 due to the engine weight. I had lightweight starter and other accessories however. The plane needed yaw stability with the floats so Rather than a central fin I made Finley’s that fit the outside I of the horizontal stab, replacing the hinge plate. My cruise averaged 100 at 2300 rpm. Zenair are great floats and I would recommend the amphibs. Talk with Michael Heinz for more info on incidence of angle of floats to aircraft centerline, this will greatly effect speed. Get it wrong and it creates lots of drag! Have fun and good luck. Dennis

You are pretty close with weight calculation. With the O-320 I put 20 pounds back by the tail to get a center CG. I ran Sensenich metal prop.