I have a Goldberg Anivesey Cub on Great Planes floats (60 size).   They work well.   The stock mounting location for the main gear on the float and cub provide good alignment to position the float step in the correct place.  You are correct that the rear mount can be attached to lower rear cabin area on the fuse.  Simply angle the rear support to get the correct height/angle of attack.   I used the supplied nylon straps and some small wood blocks in the back of the cabin floor.  The float kit comes with an extra rear dorsal tail.   I never mounted it and never needed it.  Changing from floats to wheels is less than a 10min task.   It's a great flying plane off floats or wheels, you will enjoy it.