RE: OS .75 AX vs. OS .61 SF ABC
I've had 2 OS .61SFs, 1 ABN & 1 ring, and also a .46SF ABN. Both ABN OSs had the liners peel. I still have the OS .61SF ring and have used it in at least a dozen airplanes. I would recommend a castor blend fuel in the ABN engines. I haven't had any trouble with my OS engines after I started using castor. I also set for a slight smoke trail after takeoff.
The 75AX is a horse. I don't own one, but I've seen them. Don't think you need a small 12" prop or even a 13-6. It will turn a 14" prop just great with tons of thrust.