Some Float Issues
The power you have is entirely suffficient for that airplane even if it may be a little heavy although you never said how much you added by abandoning the GP struts.
Always difficult to tell from pictures but from the photo and description of the takeoff it appears you have inadaquate float incidence. The wing must be at three or four degrees positive incidence to the float top decks in other words when the airplane is sitting on the bench and the top deck of the floats is proped up level the tail will be slightly low with the wings leading edge raised slightly.
What happens is the airplane will accelerate to a point on the step just below flying speed and as you try to rotate for take off by adding elevator to increase the angle of attack of the wing the heels of the floats will hit the water and just put on the brakes preventing take off. Now you are motor boating along just below flying speed and if you hit a little wave or you force it off with full elevator it will bounce into the air with to little speed and she stalls and rolls over.
The GP struts setup is actually a good arrangement and it make it very easy to make small float incidence (float decaladge) adjustments at the lake by carrying some simple plywood spacers under the wire clamps and adjust the incidence.
John