My body guy is finishing up a 71 Cuda that he has built from the ground up, and used a 3" exhaust, but instead of using regular mufflers, he used some kind that actually mount inside the exhaust pipe. I believe he had them tacked in just before the turn up over the rear end, so all that runs out the back is the pipe itself. His only problem is that he has to fab up the box tips out the back, because he couldn't use the ones he had. He got this idea from the Sprint car that he runs. If you think this might work for you let me know and I can try to find out what mufflers he used.