Its hard to tell from your pictures, but the center float looks as though it doesn't extend far enough forward. Check out the following link:
http://www.warbirdpictures.com/NavyBWZeros/Rufe-4s.jpg
Look at where the prop line is on this plane; the float length in front of the prop is at least the length of the front of the wing to the front of the cowl. Perhaps you should add an extension to your pontoon.