The muffler on the car now (67 Monaco, so a C-body) likely dates to 1987 (car wasn't driven, stored inside between 2000 - 2022). I don't know where I'd find the size/dimensions for the factory muffler.
I've downloaded a 400+ page PDF file from Walker. I can scan down the tables and find sizes that are close. I'd prefer stainless, and I think I'd prefer their QuietFlow and not the SoundFX.
Here's a question: Is muffling proportional to the muffler size (or internal volume) ? I have room for a larger muffler. I also know (or I think) that hanger design/construction can play a huge role in cabin noise.