Steve, the Turbo Pros are pretty solid duplicates of Super Turbos and they work pretty well. Not sure they sell one that is 18" long offset center but they do have a 14" version and an 18" that is offset offset.
Instead of replacing mufflers, what if you installed a resonator? There is a lot of good information out there about what the resonator does to the sound.
Thinking of doing that to my Dart using resonators for 1965 Dart with Commando 273. Just an idea.