I have a set of foam/balsa floats I built for my 4 star forty and want to use them on my brothers falcon 56. His plane ready to fly on land without fuel is exactly 5 pounds. Both floats without rudder and mounting hardware are half a pound. The fuse on the 4 star was longer so the floats are 35.5 inches long and the falcon is 42 long from prop plate to hingeline. This makes the floats 85% the length. I read an article that the floats should be 75% but not more than 80% the length. Will these floats work?

Yep.
 Just rig them properly.

 All the float "Rules" are just guidelines. 

And a little too big is always better that a litte too small,,, at least that's what they tell me

Thanks guys, Another question I have that I have not searched yet is "are 1/4" nylon bolts ok to hold the alumunum mounts to the airplane or should I use steel?

1/4" nylon bolts are fine, and the slight amount of ovality in them greatly reduces the chances of them loosening with vibration.  If you crash hard enough to snap the Nylon screws, you're going to have major airframe damage anyway.  Stronger steel screws would just ensure that the landing gear struts are even more damaged. 

I usually hold mine on with #8 steel screws and tee-nuts, but only for the convenience of the tee-nuts.  Nylon-insert locknuts are really approriate for  securing the float strut pieces together.  I have landed  with a float dangling and the plane survived, but it made my knees all wobbly and I would prfer not to repeat that experience. 

Well thank you for all the info. I had 4 take offs today and 4 landings on the floats. The plane flys very well in the air, rolls are smooth without much drop and it flies upsidedown with no input. It will even knife edge. I will try it again tomorrow morning on smoother water