I ran a WM P-51 with a 46FX like yours, fun plane, weak landing gear.

Reading your RPM's it sounds like you still might have the baffle in the muffler. I would recommend the 9.5x 7.0 APC prop. This series of props will do better on the top end. It should get the motor to spin in the 15,000 range with out the baffle installed. A 7 pitch would still allow good pulling power to get it off the ground and give good top end for what I would assume as a 5 or 6 pound model. You may have to contact APC directly if your LHS can't help.

I also had one of those tower mufflers which helps a good deal. But you need a spacer between the muffler and the motor to help clear the cowl. Tape up that canopy and go fly. Good luck