well the engines are just not that strong for turning props. They sell them by hyping up the horse power numbers at high rpm so people think they make awesome power. That engine will need a 8x5 prop to turn 20,000 rpm. Your choice to order a Jett 35 is a very wise choice. One thing about the Jett 35 engines though is you need to stay on top of the rear bearings need to change them every 2 gallons of fuel or so to stay safe. The case design along with the crankshaft size has the bearing on the small size. This coupled with the power it makes has them wearing out faster than normal. I run the Jett 35 LX rear exhaust engines and they scream around 21,500 on 8x7 APC props with the marine pipe.
here is a video of my old Jett 35 in a LR1 plane from hobby lobby. I didn't know i was being filmed anyway here is what one can do.
https://www.youtube.com/watch?v=jYFfBags08M