Hi Steven,
As I understand it, the early 113's are affected by the exhaust pieces off the manifold. I have stainless from Timevalve all the way, but the fit is tricky. Joe A. did mine, and he asked Timevalve to send him the three sections unwelded, so he could make them fit properly. He welded them in place, and the fit is perfect. A full day job though, at least.
The tips from George at Authentic Classics are amazing. You may have to put crimps in the ends to make them fit firmly on the end of the stainless pipes.
Hope that helps.
Mike Mizesko