1965 Mustang, How to quiet down interior noises

I have a pretty stock 65 with a 289. I use the car as my daily driver. a long time ago I installed some noise canceling insulation under the carpet, and behind the rear seats. this quieted the ride quite a bit, but the material I used was the cheap hardware store type. I couldn't bring myself to spend the money on the expensive stuff. Now I am looking to improve on quieting the interior and I question just how quiet can I get this thing? What would keep me from getting it as quiet as a new car? is that possible? Do you think my results would be much improved with the expensive automotive style noise canceling insulation? because the car is running and I don't want to pull the whole thing apart, there will be places I cant easily get to add insulation (ie. the Firewall) I'm wondering if leaving something like that uncovered will totally negate the use of insulation everywhere else or if it will only have a proportional effect? What have you guys done to block noise? any suggestions? Thanks for any advice