I am also interested in adding some type of sound deadening mat in my car. I searched for a thread I started sometime ago regarding the same thing, but gave up when I couldn't find it.
My 70 Charger is fine for short trips but once you drive for more than 45 minutes to an hour, the noise and drone tend to wear on me. I am leaning heavily toward Fat mat. Amazon sells a roll that is 1 ft wide, 50 ft long and 80 mils thick. $93. I figure that 2 rolls will cover the whole floor, package tray, trunk divider and behind the door and kick panels. If it only reduces the interior noise by 15-20%, I'd consider that a win.