Here is a pic of the Aero Turbine muffler, mounted under my RV-8.

The distance between mounts is 15". I removed the front seat and the floor between the front & rear spars, which allowed enough room to rivet in eight -3 plate nuts. These attached to the bottom fuselage skin where the flanges of two longitudinal frames made natural doublers. The Ford type hanger brackets seen in the picture are bolted to these plate nuts.

Perhaps the muffler is located somewhat aft of the ideal aesthetic position? It was a matter of practicality.

The exhaust pipe ball joints are inexpensive 304 stainless pieces which have limited, but enough angular movement and a lightweight clamp. I hope these clamps work out, as they offer light weight & drag. If they move around and look like they might fail, I'm considering hooks & springs, offered by Burns Stainless. 

Total weight of the new exhaust: 12 pounds.