Check out Sound Deadener Showdown if it's still around. His theory, supported by testing, is you only need a small percentage of the stick on butyl to take the bong out of the sheet metal. Then a layer of isolating foam, then a layer of rubber.
Must be something to it since the manufacturers like Dynomat are selling the other materials.
Sic friatur crustulum
'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.