As I recall, the '72 Imperial single exhaust uses 2.5" inlet/outlet pipes. Allegedly same flow restriction as an OEM Street Hemi muffler, but in a C-body size "case". Upgraded my '67 Newport 383 4bbl factory single exhaust system to that muffler and pipes. Had a small issue with the ball joint connection at the front, plus had to get the rear resonator-eliminator pipe bent upward to match the lower quarter panel angles of the '67. Sounds stock, as it should. Sourced it all from Walker Exhaust, back in the late '80s. Might check an online NAPA Exhaust catalog for the muffler specs (should be online) or possibly an online Walker Exhaust catalog (will probably show everything "NA" except the muffler, clamps, and possibly the mounts.
Just some thoughts,
CBODY67