On a '67S motor with Webers I had good results with modifying a stock muffler according to the old Up-Fixing article, but leaving a single outlet rather than dual. We also cut out the perforated section of piping in the section where the holes are torched in the dividing bulkheads to allow both inlets to share the single outlet better. I have a pic on my website of the original article which shows a couple of cutaway drawings if anyone is interested in a reference. This is my first attempt at posting a link to this board, though, so I hope it comes out OK.

members.rennlist.com/tweedt/sptmuff.jpg