65/66 AC and non-AC firewalls all all started out the same with no cut-outs, holes etc. Then they were stamped for cut-outs depending if heater only or factory Air. The major differences are the larger hole for the blower motor, the holes for the AC lines, mounting holes for both units and the electrical connector is horizontal rather than vertical (the AC part interferes with the vertical connector). Can a 65/66 non-AC firewall be converted to an AC firewall - sure. Take lots of measurements, have a template for various holes and drill. Rather than try and cut out for the larger blower motor, I found it easier to weld the donor section in my 65 Dart non-AC firewall.