Im trying to put a 15 or 20 gasser in my new kit i just bought. The kit calls for a 91 supass and a 15cc gas is only 3 ounces heavier also reading other chipmunk kit builders they have to ad nose weight when balancing the plane. am i crazy or will it work?

I'd do it in a heartbeat

I would give it a whirl. Sounds intriguing. The one I built has an OS .61 FX with no additional weight added to the nose.
http://www.youtube.com/watch?v=9PFnD2gpMi4