Sound Deadening?

Thanks to all. Yes, I live in NW Iowa where the temps can range from 25 below or colder to 110 above in the shade with some crazy humidity. So if is not just the salt on our roads causing the rusty cars and trucks. My concern is to deaden the sound, and allow the metal to breathe as well.....am I right? I did not do a great job on the 66GT, and i know/hear it. The 65 will be different for sure. Thanks again. Old Dart 66.