hi AVIATOR135...
I have a VMAR Mentor T-34, put a VMAX .61 swinging Bolly Prop 11.5x7, it's fast... I have also an RV-4, same size as your Harmon, but I put GMS .76 (2 stroke) with 11x8 Scimitar Profile Master Airscrew, perfect pair! Try also 12x5 prop for your .61 engine, and feel the adrenalin rush!!!