I fly an LT-40 on floats and have a 46 engine in it . I have used both Great Planes floats and Plane Fun Floats and the engine is just perfect.
I do not know the weight of any of the floats but did not notice any appreciable difference between flying with wheels or floats.
The GP and PF floats both need some type of covering. I covered the GP floats with Monokoat and I glassed the PF floats. The GP floats did leak a little . That's one good thing about the PF floats --- they won't leak.
You can buy some 1/8 x 1 aluminum bar stock at Home Depot which makes great mounts for the floats. If you need I will post pics of how to do this.
Good luck and enjoy the water --- Ray