I went with foil type bubble insulation on the floor and rear wall. The fire wall I purchased a pre cut pad for. Then I put down the ultra plush carpet with the mass backing. I covered the floor, door bottoms, front kick panels and behind the seat (in front of the tank).
Not only was I concerned with sound, but keeping the cab cool in the heat. On 95 degree days we have to turn down the A/C fan ‘cause it gets too cold.
Here’s where I got my carpet – and it fit like a glove.
http://www.stockinteriors.com/AutoCarpet.asp?Itemid=374&MakeId=31&ModelId=88
