The american thunder serie flows ARE the 40series 2 chambers.Very low pitch rumble @ idle and that classic V8 roar @ wide open throttle.I know this cause I havem on my '94. Bro hope this helps yah and maybe u can get some sleep now huh? Aloha from Hawaii! :headbang:

 

I just threw the flowmaster axleback exhaust on my '11 mustang 5.0. Sounds very good. I know this doesn't answer your question but just thought I would give you a thumbs up on it

 

I checked with Flowmaster - this from the R & D folks: It really depends on the individual kit. There are quite a few American thunder systems that use Super 50 mufflers, 80 series, etc.
So the answer is - maybe - the kits are designed for power and sound enhancement. The mufflers change with the application. Hope that helps.