When I made the upholstered panels on the back of my cab I used a 1/4" sheet of luan, and some light foam under the material. I only went down below the top of the seat back. That made a big difference. Anything you do to soften up the interior is going to cut down on the noise level (door panels, kick panels, headliner, etc). Like you I don't have anything on the firewall yet, but plan to use something like the dynamat product-especially on the roof to cut down on the heat during summer shows. I've got rubber matting on the floor which also helps, but because the windshield gasket I got from Roberts leaks in the rain, I haven't put any type of backing under it as I don't want anything that holds water-but it doesn't see much rain locked in the garage. Mike