For you to make it worth doing, in my opinion, you have to almost strip the entire interior. Carpet ,console,seats, heater box,kick panels, all of those need to be removed and treated for it to make it worth the trouble. I did my underside roof inside the doors, firewall, floor, and kick panels. I used the 2 step "sound and heat" lizard skin and then covered with dynamat. Don't know you will notice much difference doing just the firewall. Floor gets hot too.
1969 x-77 Z/28 orange/ white stripes, named by my daughter "Clementine ", M-22 close ratio, 4:10 12 bolt, CE 302. Hugger orange with white stripes. White standard interior. Restoration complete May 2018