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I don't know about A/C cars, but I changed the one on my '77 from the engine compartment. It is on the passanger side firewall. Detach the hoses, take out the bolts on the cover and remove the heater core. You might have to cut out some of the insulation and then angle it out.
Either A/C or non A/C, check the size of the new core to the old. On my 78 the replacement core was about 1/8 inch taller and after struggling to get it in the box,(wouldn't fit) I realized the difference.
Needed to knock off the ends of some of the screws protruding into the open box to get the new core to fit in.