Sig piper cub 71"
I would be looking at the saito 56 four stroke. Anything less and you won't get it off the water most of the time. We use the surpass 48 too, but that has more trouble getting it off of the floats. You can always cut the throttle. Don't put in too little, you will hate it.
Everyone here that flys them on floats has moved up to the 56.
Remember, the box reccomendation is for 2 stroke engines. With four strokes, you need more.