I have a '67 with factory air that needs a new heater core installed. How do I remove old and install neww. Do I have work from the engine compartment, interior, both? Do I have to remove the fender? Please send help. Thanks.

You have to disconnect hoses and remove some nuts from the engine compartment and remove the core from inside under the dash. To access the hoses and nuts there are two methods; cut an access flap in the inner fender well or remove the fender and well. Here's some earlier discussion on it. /forums/ultimatebb.php?ubb=get_topic;f=23;t=004294#000000
and
https://firstgenfirebird.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=23;t=004599
A dealer cut an access "door" into mine in the 70s to get to mine. Since, I have reopened it again to replace the core. I have a few small strips of metal and screws keeping it closed. I then keep silicone caulk smeared smoothly over it and then coated with undercoating so that it is almost invisible.