1972 Duster. Are firewalls different for heater/defroster models versus A/C ?

It was a lot of work going from non ac to ac, I haven’t tried going the other way. Should be a lot easier. The biggest issue is that the ac fan hole in the firewall is much larger than the no ac hole.

Might be easier to just leave the under hood ac stuff off and use the same heater box inside. You could leave the ac evaporator coil out too.