Just ordered the vinyl floor mat. It sounds like a few folks had good luck with this one:
http://www.stockinteriors.com/molded...=31&ModelId=88
Brothers had a rubber mat, but it didn't go back under the seats and tank. I needed something that covered as much as possible.
I've gotten ready for the mat, by putting in the mass loaded vinyl. Seemed like the right thing to do. I first taped the seams and applied POR 15, then measured out the vinyl and put it in. It helped to put a few halogen lights under the truck, as it was in the negatives when I did this. Heating things up helped. Made it more comfy, anyway. This was one of the more enjoyable chores. The roller really helped.