In our then-new '69 Chevy pickup truck, under the bare carpet were some patches of a tar-like sticky material covered with a thin layer of white plastic fluffy material, two separate items, but worked together. The white item similar to what is between a brake backing plate and the rear axle housing for sound isolation functions. That GM stuff is in repro at some of the GM pickup truck repro vendors. The DynaMat-type stuff will work better, I suspect. Plus available from lots of places.
CBODY67