Sarge, you wouldn't be misreading that manual somehow, would you? A 10-9 is a 60 size prop, or a 50-size racing prop. Your GP 42 would have been turning only ~10,500 rpm with a 10-9 --- waaaayyy below its real-world 14,000 rpm torque peak and 16,500 rpm power peak. You're going to see a huge jump in performance with the 10-6 (it'll turn ~ 13,000 rpm). If you need even more pull and a slower landing speed, an 11-4 works really well on that engine in a trainer application.