Dan with the step that far off to the rear you will never be able to rotate for takeoff.

For that airplane the the step should be either at or no more than 1/4 inch aft of the CG. The CG with the floats should be in the same position as when flown on wheels. The floats should be roughly around 75% in length of the distance between the tail and prop. You need to move the floats forward and make struts as neccessary. Using existing wheel main gear is not always a great idea.

Also the floats should be installed so that the airplanes wing has a few degrees positive incidence to the floats top deck. In other words the tail sits slightly low.

After the floats are properly installed and the CG may have changed a little you can move the battery a little or if weight is neccessary it should be added to the floats themselves and in that way you do not have to fool with the CG every time you switch back and forth with wheels.

John