I bet they are too short. Width-wise, they are probably ok, but need to be extended and then mounted to get the step ahead of the CG.
I have used the cub floats unmodified on the Bushwacker and that went well. I have recently used them, but extended, on a Legacy, but didnt make them long enough and the floats dig in under thrust. Google image search RC FLoat Plane design and you'll get the idea.
Float Design