The ideal prop for that is the 12x8 or 13x6. I noted that the engine sounds very different with a 13x6. Put one on and see, it has sufficient power, you can try the 13x4. I had the same on a Reactor which weighed 5.8-5.9lb and it didnt have the power to hover. Now I have a 75ax on that airplane.

The 300SP is much smaller and hence lighter than the Reactor so I feel the 75 would be too heavy. In fact I had to add a lot of weight (28-30oz) in the tail to balance it so I will not recommend the 75. Put a 72a and try it out, its much lighter, even than the 55ax. Alternatively, you can try the new 65ax, its cheaper than the 72A. But based on the reviews I have read, the 55 should be enough.

In the articles section there is a review by Steve Herlacher on that airplane. May be you can drop him a PM. Thats how I started

Ameyam