Yes the floats and mounts add a little extra weight so you want to look for the lightest set and rigging you can. The trick is to get the floats set-up correctly with the plane as far as aligning the floats step in reference to the CG of the wing and the floats incidence with the wing. Once that riddle is solved and the floats and rigging are not too heavy almost any high wing plane will get in the air once the plane "gets on step". Sometimes you have to experiment with different props too .
I just think you would have some trouble getting a 61 size engine in a 40 size plane and have any cowl left although I'm sure it's been done before. It really depends on the plane, size of the cowl, etc...