Lynn,
I had a computer crash (and a brain crash ) after a bunch of interruptions while trying to put this whole thing together for you. So please make sure to go back and read all that I just reposted. It will probably make more sense than the earlier gobbledy gook I wrote.
I like the idea of your rigging mock up. I'm not that smart, I just crawl around under the airplane while it hangs mid air from a chain while I cut struts and cables (see pic below and note wife's comment "Are you sure this is safe?)
Your 18 inch spacing between the fuselage and top of float would probably be ok because you aren't running a long prop.
But also remember that whatever throat angle you use will lower or raise the tail accordingly. And it's amazing what an inch or two off a strut can do to tail height. The original rigging I got with my used Aerocets were modified for a VW powered Kitfox , so it had a HUGE throat angle. After I figured out what I needed for struts and I got them installed the tail came up a about a foot and a half! I'm lucky I have a tall hangar...
(of course a guy can always build a cart or two to go under the nosewheels to keep the tail down when in the hangar)