You could try a 7x3 or a 7x4. The APC has wide blades and typically offers more load not too mention the weight of the prop. I would try wood, older Master Airscrews with medium width blades. If you can find the Cox 7x3.5 that's another option. Ideally, the engine can run a 6x4-8x4 but in my opinion a 8x4 is asking too much from that engine . Master Airscrew did make a 8x3. Both your fuel choices are fine and you may want to use 15% if your power isn't up.