Here's a few more photos from my 74 Rallye Charger.
The first two show the composition of the old sail panel insulation board. They have a thick paper board on the back, a tar composition in the middle and a thin paper on the top and is fragile due to age. The 3rd photo shows the wheel well sound insulation on the wheel well that I believe was referred to in the original post about at the sides of the trunk divider board. It is about 0.125" thick, asphalt/tar paper composition and about 24" long and 3.5 inches wide. Also fragile, hence the duct tape holding it together It's put on wheel well with rope caulk. It's the first time this car is apart. I could not find any reproductions of these panels so I made my own and kept the templates I made from the originals. My panels I made from my templates fit perfectly (better than the sagging originals). My other Charger has the divider board and if I can, I'll remove the rear seat to see if it has insulation. It's right at 32 degrees out (car is outside) but I do want to answer the question asked as posted by 340Cpe.
Terry