I have a feeling that your propeller choice will be a big downer as well.

As for moving the air intake inside the airframe I doubt you'll gain what you're looking for. The three noise makers are exhaust, airframe, and propeller. I've watched planes fly where I could not hear the engine or propeller because of the drumming of the airframe. That was a U-Can Do with a Saito 1.50 on the nose. OTH, with a well designed muffler and airframe I've stood next to an airframe where the only noise you heard was from the propeller. I don't think we want to spend that much to reach that goal, though.........

I'm with Dick on this one. Look for ways to make the airframe more rigid, experiment with some props, and use big cans.