I have used a muffler intended for a mid-1970's Cadillac on several projects. It's got 2 1/2" diameter pipes at both ends, and one muffler is rated to carry the volume of a 500 cubic inch engine. A set of duals using those mufflers has virtually zero back pressure, and they're VERY quiet!
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!