Your mathematics are sound George and using that approach will certainly keep back pressure to a minimum. But I think it is doomed to be noisy.  That's just my understanding about how mufflers generally work and I could be wrong.
 During a test of mufflers at a rotary fly-in some years back I was amazed at how effective the muffler became and how small the power loss was when we installed a 1" diameter muffler outlet pipe on the test stand engine  (13B).  1" is admittedly too small but we learn by going to extremes sometimes.

Tracy