I tried power reading each reply but did you determine the initial timing?

Higher timing at idle will reduce the choppy idle.
Why stop at 15? because of your starter? Run it higher. I read you have enough vacuum for power brakes. How much vacuum are you pulling.

Running manifold vacuum advance is another way for achieving big timing numbers at idle.

How about a looser torque convertor?

A bigger exhaust system is another way to mash the sound.