Not sure why the issue? A B/RB engined C-body muffler has been a C-body muffler from 1965 to 1973. They are all the same size, single and dual exhaust, except for the 1969 Dodge Polara 440/375 pursuit cars, which used Street Hemi mufflers under them that ONE model year.
Now, although the case size is all the same on the C-body mufflers, except the '69 Polara mentioned above, the pipe sizes can differ a little bit. The '72 Imperial muffler has the largest pipe sizes and reputed to be very similar flow restriction as the Street Hemi muffler.
After I got my '67 Newport 383 in 1981, it had the factory 4bbl with a single exhaust (although similar Plymouths and Dodges all had dual exhausts with the same 383 4bbl engine). Horsepower ratings were the same 325 horsepower for all. So, with a muffler shop muffler and tail pipe, I wanted to put it back correct and ordered up a '72 Imperial y-pipe back system from a Walker dealer. I had the full Walker paper catalog so that project was fully researched before it began.
So the Walker muffler claimed to have some absorbent material in it to keep it from deteriorating internally from moisture, plus having the small condensate drip hole at the rear seam on the bottom of the case. For the tail pipe, it was '72 also, without the resonator.
So as the middle of the body is pretty much all the same, the pipes fit perfectly and the contours were correct too, as expected as Walker is an OEM supplier, or was back then. The end of the y-pipe, at the ball joint connector, was pitted and might not seal well, so I initially got a tube of Walker's "muffler cement" to fill the holes, but it still leaked just a bit. BUT . . . I figured out a way to fix that!
When I was doing this deal, Chevy had started to use a composition exhaust pipe seal at the exhaust manifold-to-exhaust pipe joint. It was basically 1/2 of a doughnut gasket, but with a metal retainer which snapped into the menifold. So, checking the sizes, I found one of those which was just right to adapt the y-pipe size to the intermediate pipe size of the Imperial pipe. Snapped the metal retainer into the y-pipe and the composition material mated perfectly with the intermediate pipe. Only added about 1/3 of an inch to the total assembled length. All was well.
At the rear of the car, the quarter panel bottom on the Fuselage Car is pretty much parallel to the road surface, so the rear pipes follow that orientation, due to having to clear the rear valence panel below the bumper. Whereas on the 'Slabs', the rear quarter panel angles upward a bit to match the level of the rear bumper. Plus, the '67 had a Class I trailer hitch on it which was in the way. So a trip to a muffler shop which could put the rear pipe in a bender and tweak it up about 1.5" was in order. Success!!!! The "sewer pipe" single exhaust system was completed. Using the existing brackets and hangers. And nobody knows what's under there, either!
At this point in time, probably about all that Walker still has available is the muffler and some hangers, I suspect. NAPA also carries Walker Exhaust, even with the Walker numbers on them . . . or used to. So another source of the complete y-pipe back system might need to be investigated, at least for the pipes.
Personally, I like OEM factory-configuratioin exhaust system items that I can install in the driveway (or in my shadetree shop) with the car safely on jackstands. Seems like everytime I tried to use a quality muffler shop for something, I always ended up getting the OEM stuff to replace it, fwiw. Pipe bends that hit something, quality of the pipes' metal, or whatever. OEM was just better, back then. And to me, that meant Walker Exhaust. But in our present world, I might need to diversify a bit.
Seems like the Formals have used a similar-sized muffler case, but with a notch larger pipe sizes? But in the same inlet/outlet configuration.
Just my experiences,
CBODY67