I'm in the process of adding factory air to my '67. I'm about to install the heater/air duct assembly under the dash to the firewall. The new firewall pad has a rectangular cutout where the evaporator outlet is, but looking at the AIM (C60 A6), it looks like I should cut out a much larger section of the firewall pad and mount the duct assembly directly to the firewall with a gasket. Am I reading this correctly? Thx!