1970 Duster A/C Firewall

RedFish said
Just FYI... A modern aftermarket a/c system added to your factory no a/c build will put you waaaaaay ahead in the grand scheme of things. The OEM a/c units have problems scattered throughout. More so in pre 73.

Will the aftermarket units use the factory A/C firewall or are they be designed for the non-A/C firewall w/ modifications?