RE: Great Plains Extra 300 S build
I realize you are using a 60 sized engine, but what size is the Extra, .40 or .60? The reason I ask, is that I built the .40 size from the kit but put a .75 on it and made no modifications whatsoever to make it work, balanced fine. Fastest plane I have and climbs out of sight quickly, but I am just an intermediate sport flyer as well.
As for the plane itself, I have seen the .60 in action and it flies beautifully. I have to say that the .40 that I built was just as good. Mine built pretty light to my liking, even though I tend to build heavier and a .75 2-stroke on board. I say, whether .40 or .60, build it as the plans call for and enjoy. You will have a great flying plane!