The 9x6 prop will work fine. I might also think about 9x5 as well. I use APC almost exclusivly on my planes. I have found that you can get about 1000 more RPM if you run the webra mini tuned pipe from the Pylon/Quickie .40 engine. I had to drill another hole in the muffler for the smaller spacing of the exhaust holes and I used a plug in the outlet to increase the back preasue. I am running this on a fun fly plabe and it screams! I am running a APC 10.25 x 3.5 or something like that at over 15K.
Sparky