Which 3" tailpipes?
I have a 66 Dart that I bought the TTI headers and the 3" x pipe. Those parts fit great. My springs are moved in 3" and when I called TTI about their tailpipes I was told that their 3" tailpipes would not work on any car with a spring relocation kit. I just made tailpipes yesterday using pieces and welding them together as I went. I ended up with them exiting just behind the rear tires. As for noise there was not a lot a difference with just mufflers then going to the tailpipes. If you want more detail let me know. Also if you decide to make your own pipes I found a place in maryland that has mandrel bent pieces that are much less money then buying from summit or jegs. Happy to pass that on as well.
Best of luck
Rod