I applied more sound deadening to my 68 this last year than any human likely ever should. I read the sound deadener showdown and researched just about all the materials. I ended up going with the Damplifier Pro as described in the showdown. then sound absorber over that. Overkill pro foams etc. I found the stuff pretty easy to apply with only a few sliced digits from the metal layer. stuck a few quick pics up to show the madness. My kids said it looked like an "easy-bake oven" for a while. Second Skin very good people to deal with. Although they should be for the amount I used.
Sound Deadener pics