[70 Rick]

I am going through this right now on my '70. The original material was made from jute, the same stuff some of the rear package trays are made from. Jute is an organic material, which is why it decomposes into nasty dust after about 30 years. I believe most suppliers do not reproduce this in the same material anyway (its usually a form of spun poly) so it won't be like the original. I decided to go with a more modern sound deadener like Dynamat or Raamat for this reason. Also, you might want to avoid using Peel and Seal or similar products that use tar instead of butyl for the substrate. In extreme heat they can come unglued and will also give off a nasty odor. This is a little dated, but lots of useful info here on sound deadeners:
http://www.dctra.org/files/1974_TR6_...r_Showdown.pdf