The horizontal crossmembers are called spreader bars, although they SHOULD be called anti-spread bars. Their function is served in tension, and they can be quite lightweight. With dual Dural Landing gears, I would wager that you could dispense with them entirely. Put 'em in later if you notice that the floats are dancing over ripples.
With a floater like a SrFalcon, you won't spend any time on the water once you open the throttle anyway.
Save a buck or 20 and get the 3/4" x 1/8" aluminum stock at home depot. Bend it into the appropriate shape and use spreader bars, since it is softer than dural. Lighter, less expensive, and you can bend it to juuuust the right size to ensure the proper incidence angles without a lot of shimming.