A/C system 67 barracuda

The standard firewall can be modified to be a a/c firewall. You're removing metal at blower motor, cutting/drilling holes, etc... There's at least one hole I recall that was stamped bumped out about 3/4 inch so you're gonna need spacers, different hardware, whatever to make it work.
There once was a sample firewall section floating around, mailed from one owner to another, just for this. I don't know if its still available. Only positive note... Changing a factory a/c build to standard basic heat/defrost is more difficult.
The correct under dash vent assembly has chromed pot metal louvers which are often pitted and/or broken. A super nice one wont be cheap. The vacuum/electric switch block for 67 push button controller is different some kind of way in the wiring and I think others have fingered out how to make the 68 up type work. The correct water control valve is over complicated with 2 vacuum supply lines and there's probably a way around that too. The V2 compumper is monster designed for r-12, and the more you learn about the 50 year old factory a/c the more you'll like new aftermarket kits. Good luck either way you go.