Use a GWS 370 C (5.33:1) and a 2 cell 1200 etech lipoly pack and a 10X4.7 prop. It will have all the power you need, and can also be run with an 8 cell 720 nimh pack. Don't forget to add the extra servo for rudder control. Inverted flat spins with an IFO are great!
Mike
www.michaelconnally.com for videos