My 1970 Duster 340 FM3 project

Adding factory AC would require changing the firewall. On a matching numbers, rare color car like that.....I would just go with an aftermarket system to avoid hacking the firewall. You will get better air conditioning, and all of the hoses for both heat and AC go thru an adapter plate that bolts in where the factory fan motor used to be. If for some reason he ever decided to sell the car, a new owner could easily remove the aftermarket AC and put the heater box back in if he/she was :realcrazy: enough to not want AC.