Sound Proofing a 126

I have been looking for a way to make the interior of my 83 300SD just a bit quieter at speeds in the 80 mph range since I spend a considerable amount of time traveling at those speeds . I have searched plenty before attempting this project but almost all searches ended up with no real information or half baked attempts due to lack of time, finance or both. Hopefully this post will answer a few questions for anyone wanting to attempt a similar project. The following is a list of what I have done so far.
Removed seats, carpet, console, rear shelf, all factory sound proofing that was loose and falling apart.
Installed butyl mats from secondskin audio http://www.secondskinaudio.com/ (damplifier pro) everywhere it was humanly possible to reach. "painted" over mats with a visco elastic ceramic impregnated vibration and sound proofing coating, also from 2nd skin (firewall). Removed factory hood pad and "painted" underside of hood with "firewall" coating, reinstalled pad.
This was a 3 day project and was very effective at almost eliminating all road noise but unfortanately has done almost nothing to control engine noise at these speeds. It did however almost eliminate engine noise at 2000 rpm and below.
I have ordered a thick aluminized fiber based pad that is used called "the insulator" from Harold electric http://www.heco.net/noise.html. This stuff is used to quiet the noise from diesel buses and motor homes. I am planning in installing on the exterior of the firewall from the top of the firewall where it meets the hood down to and along the trans tunnel and attaching it via self tapping screws with fender washers. I have much higher hopes for this pad and will post results after completed.
I will post pics of entire project soon.