When I got my Challenger in '80 it had a pair of old Walker Continental glasspack mufflers; cylindrical with flat seamed endcaps, not the 'bullet' type like cherry bombs. The sound (given the stock 318 2V) was incredible; mellow, smooth burbling that sounded like a much bigger engine. I really wish I had kept them... they're impossible to find now.
I have no idea how 'efficient' they were... but the sound... wow.