pjkaiser: Unlike modern cars, you can almost see the heater core because it is in a box in front of the firewall on the passenger side. However, one of the screws securing the the box cover points down and is very close to the inner fender.
I had to do this fix one time and I drilled a hole large enough in the inner fender to put a flat blade screwdriver through and remove the screw. After the core was replaced, the hole was plugged with black silicone.
An issue of the KF Quarterly from the late 1980s had a template to locate the hole.
