Non-ac car needs air conditioning

I am helping my buddy with his 1975 Dart sport hang 10

It is originally a non-ac car and we want to put AC in
I have an AC heater box but none of the holes line up in the firewall. Heater box is from a 1973 Dart sport

We are wondering is it easiest to put an aftermarket kit in if so which one rather than two hack up the firewall?.
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