Jim,
I just bought new exhaust when I re-did the 67. I had the stock...semi worn out sides. When I installed the new motor the pressure really blew the exhaust out and I had to replace it. I talked to Allen's, Sweet Thunder and Stainless works. I decided to go with Stainless works(non-stainless) sides because they told me that they would probably be more civilized. Not true, these are the loudest sides I have ever heard. I think they are louder because I did opt for the largest inner diameters that they had. I can feel the diference in the power but I would prefer a little quiter sound. It really sounds like straight pipes at certain times. At the idle the cam sounds really mean and really loud. When WOT is applied the exhaust actually quites down some. So the majority of the time it is too loud for me. If your motor is close to stock I would go back with the stock inner diameter and keep things a little quite. Stainless will have a different sound and may have a ringing affect.
Good luck,
Tim