I like to build "sleepers" and an obnoxious exhaust note is simply a way of telegraphing my punches. Whenever possible, I use the muffler that came stock on mid-1970's Cadillacs. It has a 2 1/2" inlet and outlet, and a single muffler was engineered to flow enough volume to handle a 500 cubic inch engine without undue restriction. Putting two of them on a stovebolt (6 or V8) makes it quiet, and the exhaust system has no back pressure whatsoever.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!