I have always liked Walker Exhaust OEM replacement mufflers. There is ONE C-body muffler from about 1965 to the 1973 model years. Dual exhausts had the same muffler, just two of them. Talking about 383-440 C-bodies, not sure what the 318 cars had under them. NAPA used to sell Walker Exhaust products under the NAPA name (and the same part number as the Walker items).
There was ONE year where the 440/375 C-bodies had a different muffler, Street Hemi in this case, than the normal muffler. It was the 1969 Polara police car with the 440/375 engine. Other parts of the exhaust system were the same. 1968 and 1970 440/375 cars reverted back to the normal C-body mufflers.
In 1969, the 440/375 police Polara set a top speed record of right at 150mph in the Michigan State Police Car Tests. A record that took a mid-1980s IROC Camaro 5.7L TPI to BARELY best. That particular Polara had a 2.7 rear axle ratio and L84-15 tires on it, for a bit of an overall gearing advantage of the normal 3.2 rear axle ratio, in the top speed area.
The genuine Streeet HEMI mufflers were of the same case sizing, just shorter and with larger inlet and outlet pipes. In that later-1960s time frame, there were only TWO real facctory low-restriction mufflers. The Corvair TURBO mufflers and the Chrysler Street HEMI mufflers. There were MANY imitations, too! The Corvair TURBO dating back to the Corvair Sypder turbo cars of the very early 1960s. So imitaitons of that Corvair muffler was the first use of the term "turbo muffler" in relation to a "performance" muffler, but some were better than others, as some companies wanted a "cheap imitation" to sell lots of them. The OEM Corvair Turbo mufflers were more rounded in the case dimensions, whereas the Chrysler Street HEMI mufflers were flatter, as per normal Chrysler mufflers of that time, and were easier to fit under low-ground clearance vehicles.
What I put under my '67 Newport 383 4bbl car (Chryslers with the 383 4bbl got a factory single exhaust as similar Plymouths and Dodges for dual exhausts) was a "from the y-pipe back" '72 Imperial exhaust system. Seems that THAT application used a normal C-body sized muffler that has very similar flow restriction as the Street HEMI muffler, with 2.5" pipe diameters. It all fit well . . . other than the very rear resonator-eliminator pipe and a simple adapter to the original y-pipe ball joint connection at the very front. My "sewer pipe single exhaust". At the rear, the body contours of the slabs angles upward to the rear bumper whereas the Fuselage C-bodies had a more horizontal line between the rear wheels and the rear bumper. A tweak at the muffler shop, for a slight upward angle fixed that.
In current times, it seems that the ONLY people who like the Flowmasters (the models with the fewest number of internal "flow directors") are the 5.0L Mustang enthusiasts. You can hear them coming, if you know what to listen for!
I believe that if you go into the Walker Exhaust online catalog, you can find the specs for the Chrysler mufflers (as to case size, pipe diameters, and pipe orientations). Walker was an OEM supplier and their stuff was an exact fit for the factory production OEM items. I highly suspect that your car has been "muffler-shop-ized" with a generic muffler of an incorrect length, something they had that "will fit" and is welded to the pipes (rather than using quality exhaust pipe clamps)? By observation, most muffler shops will use a generic muffler of a similar size rather than order in a correct muffler, from my experiences. Nothing really wrong with that, just that what they use is usually not exactly correct. Gets the job done. BTAIM
If not using a Walker Exhaust muffler, then MagnaFlow would be my next choice. Of the correct size and such, of course. And using quality exhaust pipe clamps so that I could change it in my driveway with the car on sturdy jack stands (or on an overhead lift), at least in my younger days. Some muffler shops, around here, were very variable in the quality of the work they did, back then.
Sorry for the length. Just my experiences.