I just finished replacing the heater core in my 64 Wildcat with AC. I found that the Advance Auto/Carquest 399062 still fits. Some others were reporting it was too short. Tom Telesco recommends using bypass or flush plugs hooked up to a faucet to test before it goes in the car. I replaced the heater core, glove box liner, and added a mercury switch for the glove box light instead of the plunger switch that always hung up the door. I've attached the video I made. It is a deep dive, with the hope to help someone else that has to tackle this.