RE: GP Extra 300 .40 Size with .61 FX
A pal of mine has one with a 46FX, and I was not impressed. A 61FX, and maybe a Bisson Custom Muffler, would be nice. I don't know how big of a prop you could get on it. I'd want a 13x6, or so, if it were mine. Also, be prepared to move a servo or two to the tail.
Good luck, and let us know how it goes.