After 30+ years, any factory sound deadening material is pretty much gone in these cars, so yeah, it will suck. It definitely would be easier to install before putting the interior back together, especially at the firewall. You might surf over to the Vintage Mustang Forum for 66 specific info. Many folks view Dynamat as an unnecessarily expensive sound deadener and recommend cheaper alternatives. Since he's going to sell it after restoration, something like brown bread or raammat may be better.