I’ve received a handful of emails in the past 2 weeks about sound absorbing behind the ’67 Beetle firewall. I highly recommend the 3 piece kit that Wolfsburg West offers. Part number: 111813805. It’s German-made tarboard and fits like what left the factory when your pride any joy was brand new. The cheaper (Brazilian) kits are much thinner and use a very lightweight tarboard. Do yourself a favor, keep it German.
You can also install sound deadening on the inside of your firewall. (Assuming your rear interior is out) This quiets things down a great bit!
My friend Chris Vallone of Classic VW Bugs also offers this same suggestion in these very helpful video tips.
If you have more questions feel free to contact me.