There's lots you can do... but it's going to cost a pretty penny and like your shop said, isn't exactly legal.
In order to change the exhaust on a 6.7 Cummins you need to delete the emissions equipment. In order to do that you need a tuner. Those are becoming harder and harder to come by now because of the EPA and their crackdown on diesel tuning. But they are still available.
If you want to start looking at tuners check out the H&S MiniMaxx or XRT Pro, SmartyJr, or a RaceME. All will get the job done. Then for exhaust you can pretty much go with whatever you want.
If you're interested in an H&S tuner I would recommend getting some custom tunes from Draconian Diesel. They run much nicer and smoother than the H&S box tunes and are also safer for your truck and blow less smoke.
Here's a thread I wrote up about the delete process on my 2012 6.7 - 2012 6.7 Cummins Tuned and Deleted - H&S MiniMaxx, Flo Pro Exhaust, and Sinister EGR Delete