Heater core r&r 66 barracuda with ac

When you say it has the under dash unit, is it just the vent or is it a “knee knocker” style. If it’s the knee knocker, the heater box would come out by undoing the J hooks and the 4 fasteners at the blower motor at the firewall. If it’s the factory heater/AC integrated unit, i’d Definitely want to consult the factory service manual. I can take a look at mine later this evening.