If I were you I would attempt to get the old heater core out and get it re-cored if possible. Newer ones are junk and not as good as the original ones. RockAuto has the heater core. This site here says they have the A/C condenser. https://www.classiquecars.com/thunderbirdaircondcart.htm. But you need to e-mail or contact them for price/availability. Also they have a heater core too. Not sure if its the original copper style or newer aluminum. As for removal. Well if its anything like the 69Tbird. I believe the heater core comes out through the engine bay by removing the cover for it. The hood also needs to be removed to get to it. Be careful with the heater core cover as after all this time its VERY fragile and can break easily if forced too much.
Replacing the heater core is a monster job that involves removing the hood, various vacuum lines, battery and inner fender plus right front wheel. Been there done that!
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