Any problems putting 3’ exhaust on 70 Dart ?

Unless you are running 500 plus horsepower, then a good 2 1/2 inch system will serve you just fine on a street car. Coincidentally, Engine Masters just did a show last week running 530 hp LS engine and the whole system with mufflers only killed 10 hp verses open headers. Granted a LS is a different animal, but it was an interesting, informative show. I'm willing to give up a few hp to not have to fight putting in the bigger pipe.