75 Dart - Stock Muffler O.D. and I.D. measurements? 225/6

Loud exhausts are fun for about a week, then they get old. Especially on an inline-six engine, a "turbo" or other "performance" muffler will set up a drone that'll cut right through you at 55 to 70 mph, right where you want to cruise on the highway. Forget it, if you value your sanity. Also forget about a stock-size muffler; the original ones were way too small for the job -- they were specced that way, together with the undersized 1¾" pipe, for reasons of cost. The original muffler was a 7¼" by 3¾" wheezer.

See this thread for ideas on muffler selection for a slant-6 A-body.