Although Ralph (Seaplane) is very high on the list of people I want to be like if I grow up, I disagree with him on the downforce thing.
Floats are a lot bigger than wheels but frontal area is not the only factor in drag. I doubt if floats make much difference in drag. Floats are a lot more streamlined than wheels.
Before WWII, the fastest airplane in the world was a seaplane, and it had BIIG floats.

I have never changed downforce angle when adding floats and never found it to be indicated.
In fairness: One of my flying buddies in FL set up a Kadet Senior with zero downthrust. that worked too.

An IggleII is a .40 size trainer. You should need floats 32-35" long. May I humbly recommend that you consult further with Ralph, since he has taken an interest in your predicament. He makes some really spiffy foam-cored floats and is developing a cult-like following here. Save the Cunningham article for later-you can always make your own custom floats when you make a scale Thrashwell-Snailby Belchfire MKIV or aqua-space shuttle.

Ralph's website is http://www.geocities.com/planefunfloats/