We have run the APC 11.5x4 on the ST45 stock. With the header and tuned pipe, pending on your envirnoment (location, temp., altitude, etc.) you may be able to get away with the APC 12.25 x 3.75. Definitely stay away from the 10X6. The pitch is too high for a Fun-Fly, and its load factor is a little on the low side (LF=26, it should be 27-28) for that motor. The 10X6 would be fine for a stock ST45 without the header and pipe, but you would not want this prop on a sport plane, not a Fun-Fly.
Hope that this helps.
Kevin