It is: as I recall, all Mopar RWD cars from the 60s to M-bodies use a different stamping for A/C and non-A/C firewalls. (The exception may be late A/C-delete C bodies, I'm not sure.)
You could always...cut a piece from anon-A/C parts car and epoxy it over the stock firewall. :wtf: (Yes, I actually saw a Duster where someone did just that!)