1968 Chrysler Newport - adding Air Conditioning.. here we go :)

The firewalls are definitely different between AC and non AC cars. An AC car has more heater hose connections at the firewall (4 connections, 3 holes, one an oval), plus the high and low pressure freon lines, and a big hole for the blower motor, and vacuum line from the back of the motor to the Heater/AC box. A non AC dash only has 2 heater hose connections and a vacuum line (Fury doesn't even have the vacuum line in 65/66), and no blower motor hole or other line holes.

However, if he uses an aftermarket AC and heat system, it's unlikely that any serious firewall mods are needed other than holes for the freon lines.
I won’t disagree with there not being differences. However, I bet that there are perforations or dimples where the extra lines belong. I would be very surprised if not.