On my I6, I have an aftermarket cam, dual outlet header, and plan to run duel 2 inch exhaust, x pipe, and Borla mufflers. Have no idea what it will sound like.
I've read that you don't want to go above 2 inch for a dual exhaust on an I6 or you remove too much back pressure for optimum performance. If I didn't have a tricked out head and cam, plus 2v carb I'd stay at 1.75 inches.
Rebuilding a classic mustang is a reality check on your individual state of perfection.