For a QUIET low-restriction exhaust, use a mid-70's Cadillac muffler, or even two of them if you've got duals. They're 2 1/2" inlet and outlet, and one muffler is designed to handle a 500-cubic-inch engine. Most muffler shops can bend up a set of 2 1/2" pipes, and you're in business without all the ear-splitting noise of flowmasters.
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!