In my 1969 Dart, the heater core was different, with longer tubes to reach over the AC evaporator, as I recall. Indeed, the whole climate box was different than in non-AC cars, and I think even the firewall differed. While in-there, check the condensate pan carefully. Mine had corroded thru to drip on the carpet. Many years later, I got a leak again, and suspected my pan repair (soldered galv sheet), but turned out I had bent the copper tube upward on the engine side of the firewall when removing the engine, so just bent it down to drain properly.
