Back in the day ('70's) some of us used to run Chrysler Imperial 440 mufflers on our Mopars. Imperials came with single exhaust for a 440 and like Jerry's Cadallic muffler had 2/1/2" inlet/outlet. The original Mopar mufflers had a big crimp in the end for some reason and we always wanted the originals.
This is a currently available option.
I ran a pair of the 440 mufflers on my 340 for a while with 2 1/2" off the headers to the mufflers and slash cut tips that exited in front of the rear wheels like the 340 6 Pak Cudas. Sound was very acceptable and at the dragstrip dyno power was about like open headers.
I do have a x-pipe on the Miata's 302 and it does give it a different sound. More of a European sports car sound as opposed to a American hot rod rumble. I like it.
Dual exhaust with 2 1/2" dumping behind the rear bumper with the mufflers mentioned above and with a xpipe (if plumbing is worked out) would make for a nice, quite, high flow performance package.
RonR
