I've insulated old trucks and cars since my first 51 3600 back in the mid 60's. My mother got an LTD and I was quite fascinated with the almost total silence and was determined to quiet my tin can truck. I first stuffed fiberglass under the dash which made a huge difference. Then I stuffed it under and behind the seat. I could drive down the highway and never hear those dang tappets rattling.
Since then I've used a bunch of different material and the truth is, each one did a better job than the last. The moral of my lengthy tale is, you pay for what you get. The cheapest will probably be the least effective. The most expensive will serve you the best. It has been my experience that there are 4 or 5 groups of insulation you can use and everyone must decide how much he is willing to spend. In my mind, insulation is one of the most important elements for a smooth ride. All that noise can make for a "LONG" trip.
