My main concern with anything actually adhered to sheet-metal, in this cesspool of humidity I live in, is the resulting corrossion that results from the constant temperature swings coupled with the water-laden atmosphere.
I would completely cover all the sheet-metal with one of those rust-proofing products, prior to adding the sound-proofing.
On another note, I have found that about 93% of noise that enters the cab does so via the shifter-boots.
I had the whole interior out of another 5-sp 4x4 truck; and, when I put the shifter-boots back on, I packed them with old carpet-padding.
It is amazing the difference in interior noise-levels this simple free fix yielded.