I have a 72 Mach I with a 302. The car is stock original right down to the orange splotches on the back of the rims. The only mod I did was to remove the single exhaust and replace with a dual exhaust via Midas about 15 years ago. The duals are true duals with no cross pipe. The pipes size I believe is either 2 or 2.25 with stock mufflers.
To me the sound increase from single to dual exhaust is fine. However, my wife has mentioned during the last 13 years or so that it needs to be louder and have more of a rumble. I am finally getting around to it because my wife recently purchased a motorcycle that has a better rumble and louder sound than my Mustang.
The guideline I am going to follow (or try to follow) is as follows:
Keep the original exhaust manifolds, the pipes from the manifold to the mufflers, and no need for a cross pipe. The car is more show than go.
With that in mind, one thought is to replace the mufflers with glasspacks with the same size in/out let diameters of the pipes. My next thought is to use Flowmaster 40 series mufflers with 2.5 in/out lets. I would come into the 40s with the smaller pipe and then replace the back with 2.5. My third, fourth, fifth, etc Ill leave up to the people on this forum that have more experience in the science of exhaust configurations.
Thanks to everyone, in advance, for taking the time to read and reply to my post.