The guy in the video wasn't quite correct in identifying the bolts sticking thru the firewall that need their nuts removed. There's one on top and 2 underneath the AC housing. The outboard lower nut is a pain to get out without removing the inner fender or getting access thru it. That's for a '78 C20 with AC anyway.
Also, once the interior ductwork is out, check the small vaccuum hose going into the middle vaccuum motor. Mine was completely shot & I replaced it while I had it in sight.