I finished the muffler installation. Per Mike Harrison's suggestion, I drilled a third (center) hole in the bracket clamp and added a spacer fabricated from 1" square Al tubing. To address the strength (lack of?) issue, I used a length of 1/4"x20 threaded rod with a wing nut to pull the center of the clamp together and gradually tightenint the other 2. Once tight, I added the center bolt and tightened. This mechanical advantage made the job much easier.
I cleaned up the burrs inside the square tubing spacer after taking the pictures.
