OK, it looks like after some actual measurements of a couple of my 71 Chryslers and Imperials that I finally have some dimensions (the cars I sampled are the ones with miles below 35K miles on them approximately and whose exhaust systems still look original). First my apologies to @CBODY67
as he appears to be correct in his choice of the Walker muffler as pretty much the same as the original ones dimensionally as he stated in a previous post in another similar thread:
Walker Exhaust 18381 Imperial/NY/Newport 440 single exh system 1972
Case length 25.0" Case height 4.25" Case width 9.75" Overall length 29.0"
Inlet pipe 2.50" offset to inside Outlet pipe 2.50" center
So what I am seeing on my 71 Chryslers and Imperials with single exhaust is that the muffler cans themselves are the same on both the single and dual exhaust vehicles - they nominally measure 25" long and 10" wide and 4.5" tall.
On the dual exhaust cars, they use two of these mufflers, one on each of the dual exhaust pipes and they are positioned just before the tailpipes start to bend upward just ahead of the rear axle and there are no resonators.
On the single exhaust systems the muffler is the same dimensions as each of the dual exhaust ones and are located in the same position just ahead of where the exhaust pipe starts to kick up ahead of the rear axle. These single exhaust systems use resonators who can is nominally 21" long and 4.5" in circumference. The single exhaust systems are on the passenger side of the car.
It seems most muffler lengths include some inlet pipe and outlet pipe, so overall dimensions of the whole muffler would be about 4" more (about 2" of pipe on the inlet and outlet of the cans). I will also post this same information in the muffler thread.