After some digging around and a couple of PMs from another poster, I've found: The stock muffler on my 67 Dart GT, 273 2 bbl, single exhaust is 18"x9"x5", 2" center inlet, 2" offset outlet. A 19" muffler would probably fit ok. 20" maybe, if the exhaust shop was creative. Stock 21" hemi mufflers...