I bought a lot of VERY expensive and VERY heavy lead-loaded peel/stick stuff intended for quieting the engine box in boats.  Used it everywhere in my 68 Westy. Made a big difference, especially that in the engine compartment.  However, most of it fell off within a couple of years, even though it included pins to be epoxied to the body. Most of the failure was internal to the material, rather than between the material and the car body.
John Collins
68 and 70 Westies in Panama
84 Westy in California