I don't know about the 91 twin on a 40size US. I don't know that engine to well, but it sounds like it would be to heavy No.1 for a 40. But for a 60 size I think it would be up to the task. After doing some reading on the FA 90T you mite want to go with another type engine, if for anything reliability. IMO....A 100 for the 60size and a 60 size engine for the 40.
I've been flying off water out in Gillman Spings, and other local Lakes at various Float Fly's. Lake Parris and Lake Casitas. I fly off asphalt in Irivne at El Toro, it's all good.