ORIGINAL: wcmorrison

My gut feel is that if you use a 12 X 6 prop you will get the ultimate thrust from the engine. Turning high RPM with a smaller prop makes a lot of noise. Using a prop more fitting the power band of the engine will produce the best performance. A bigger prop at a lower RPM more than off sets all the extra RPM (and internal wear and tear) on your engine.

But, alas, it is your airplane and power plant, do what you want with gusto. Have fun!