I think that a GMS 76 is a perfectly sized engine for great performance on a 40 sized plane with floats. In fact, all of the 40 sized planes that I had on floats flew on Tower 75s which is nearly identical to the GMS engine. On average, 40 sized floats will add around 2.5 - 3.5 lbs to your all up weight, so the extra power will allow you to have approximately the same reserve power as you would have with no floats and a 46 engine.