2 1/2" or 3"
had 3" on my small block..
used these mufflers...
this x-pipe from magnaflow
it fit fine. my springs were in stock location with a stock gas tank..
here is how it sounded. it isn't as loud as it sounds in the video and is quite a bit deeper sounding then in the video..
http://www.abodyjoe.com/pictures/71 Dart/exhaust/dart exhaust.avi
i now have 3" to the mufflers and 2 1/2" tail pipes because the 3" didn't fit with the triangulated 4 link.