The mufflers I've used on projects for a lot of years are originally for mid-1970's Cadillacs. They're designed to flow exhaust for a 472 or 500 cubic inch V-8 through one muffler. They're 2 1/2" diameter in and out, and they're nice and quiet. Dual pipes with one muffler each have virtually no backpressure, regardless of how big an engine they're on, and I don't have to listen to exhaust pipe resonance that gives me a headache!
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!