The single servo mount should be fine. You fly a J-3 with the rudder anyway. The only tip I have for the floats is that you'll need to put a block in the back to hook it to, so a rough landing wont break it out. I dont really know how to switch between floats and wheel quickly, unless you had a completely different set of struts and everything for wheels and floats. That way you dont have to remove your axels.