I put a 3" on my 71 cuda 440 with stock manifolds. It all went in fine, but had a horrible drone at 45 mph. I installed their headers, and the drone was gone. It was the easiest exhaust I have ever installed.
I had that exact same experience with my 1970 'Cuda. Manifolds with 2 1/2" pipes, terrible drone. Put TTI headers on, no drone and more power, a winning combination.
