there is likely no reason to mount them high or low, except for aesthetics.
Don't forget your mainwheels are going to be down there somewhere under, or in front of the props, and they will kick up debris.....
The FK has tons of fin area and is very stable laterally. I would just increase the rudder chord.
I was toying with a similar idea, but going one step further and make it a twin engine flying boat...it is still in the "maybe" stage.......