Here's an NOS Walker Royal Scot economy replacement muffler part #24844. These mufflers were made to replace the oval tri-flow muffler and is a fully baffled, thick walled muffler. It is a direct drop in fit. It fits the Mopar applications seen on last  photo. Also fits 1966 Mercury W/390 except wagons.

This is a Vintage Walker muffler. Just opened it for the pics and this is a MADE IN THE USA part! Not a current one size fits all foreign made muffler! THANX