Thanks to Chad and Dick for weighing in here, they are very knowledgable wrt electrics and gas engines and I appreciate their input here.

Chad - If I assume the 12" pitch goes with the 6500rpm end of the range and the 15" with the 5800rpm end, that gives a pitch speed range of 78-87mph. My example of a 10" pitch at 7500rpm or so should be OK then, considering the unloading that will take place in the air with the gas engine. Good to know it sounds like a decent starting point.

Dick - I will keep the part throttle performance in mind when setting up the pipe and I will start on the long side and sneak up on it. I will start with the 20x10 and also try a 20x12 after the engine is run in a bit. My prop calculator predicts around 6900rpm with a Menz-S 20x12 and that just might be the best all around combination. Both combinations show predicted static thrusts in excess of 23lb, which should be lots for a 2M pattern plane. Do you have any comments on the hard mounting vs. soft mounting issue? Any testing on current draw to the servos for one or the other?

Thanks to all for the help and information,
Mark