I purchased some "space blanket" type stuff off ebay -- insulation that is about 1/8 inch thick with silver backing -- high R value. It really works great and I used it on both of mine. On the '70 440, I just did the firewall as noted above (pulled the carpet back and got it as far up the firewall as possible. It really helped. On my '74, I was installing new carpet so with the seats out, I lined the whole interior of the car (including inside front door and rear panels and rear seat). This worked incredibly well -- in addition to controlling the heat -- it REALLY cut down on road noise. I think a roll of this stuff is like $25 bucks and I used 2 for the '74 and about 1/2 of one for the '70. I highly recommend it! I may have some pics if interested...
Vermont MOPAR