RE: new saito 82 questions
Hey, the 12.5x7 3-blade will look great on your RV-4. It will load it about right. I love the sound of Graupner 3-blade, as the prop sound pitch will sound much higher-pitched than engine combustion pitch.
As for the 14x4W, it peaks @ 10200 on 15% nitro, @ 10700 on 30% nitro. The peak torque of this engine occurs at around 9800 rpm. So, for 3D, I'd say use 14x6 or 15x4 on 30% nitro, or use 14x5 on 15% nitro.
This engine vibrates a lot for its size. For motor mount, a sturdy nylon mount will do. I broke the little 2-piece .40-size nylon mounts that came with my Funtana. But something like a GP adjustable mount for 2-stroke 50 would do fine.