I used some
GTMat material (don't remember which flavor) that did a good job but I just wanted to get rid of some of the road noise and wasn't trying for a really quiet cabin.
I had my favorite body shop R&R the interior panels and I installed the material myself. As per Chuck33079, it's not a hard job if you have a good cutter and a small roller.
You'll want a louder exhaust once you install the sound-deadening.