I am very happy with the new exhaust system on the 69. I opted for a 3" Pypes X pipe with cutouts, into Magnaflow mufflers and 2.5" tails out the back. It looks and sounds nice, especially when I have the caps off for an occasional trip around the block.

I have noticed that when the car gets up to temp, not just engine temp, but when the whole car is good and warm I get a strange knocking sound. One or two knocks at a time. It is not related to engine speed or vehicle speed, although I only hear it when moving. It also doesn't seem to happen over bumps or dips when the body would move. It sounds like something banging on the x pipe ! Could this possibly be expansion and contraction noise? The x pipe is pretty close to the crossmember, but doesn't look like it is making contact. Could it be that the pipe expands just enough when hot that it touches? Has anyone else ever had anything like this?

Could definietly be banging on the crossmember. You would be suprised how mych these cars flex when moving. Make sure it has plenty of clearance at rest....same goes for the rest of the system.

PS - do you have plenty of clearance with the 2.5 tails? My 3" tailpipes are a disaster. Looking to go with 2.5 pipes to gain some room.

yes, the tailpipes fit nicely, and I figured the gasses would be less turbulent and cooler by the time they exited the mufflers, so I am happy with the 2.5"s
I bought the flowmaster mandrel bent. They needed some tweeking, but fit now......