The higher gross weight for the A model was based on operation in the restricted category.
If you want to go there, you need to read carefully the "restrictions" and understand the certification standards in that category for the Cub, which basically put the limit load factor very low. In other words, at that weight, the factory said all bets are off regarding structural integrity.
This is pretty common in the restricted category.
Wipair sells an STC to up the gross weight of your cub to 2000. It's a lot better deal, even though it costs something.
I believe that A model airplanes also had no rear controls, as in the torque tube was very different. The top of the airplane was flat to accomodate the top hatch, to facilitate loading the hopper. It was hinged at the front.
As to float certification, very few Cubs were actually built as PA-18 S models, which was the seaplane version, but for all practical purposes, they are virtually all convertable easily to floats, and the FAA has been willing to approve them pretty universally.
I suspect that almost all A's have been conformed to the basic PA-18 configuration by now, for practical purposes.
MTV