Some of this you may already know. To replace the heater core, you need to remove the passenger side dash panel which will give you access to the inside A/C/heater box. The fasteners are all on the perimeter of the box. I believe there is one stud at the top of the box that goes thru the firewall that the nut is located in the vicinity of the passenger side hood latch. I believe that there is also a bolt that goes thru the firewall at the bottom of the box that can be accessed from below or by removing the side grill of the car. Once you get all of the fasteners out, you have to lift up slightly on the box and then rotate the top toward you. This will allow the heater core tubes (where the hoses attach) to clear the firewall. It's then a matter of lift and turn to get the housing out. The housing almost needs to turn 90 degrees toward you to get it to clear the firewall to come out. Once the housing is out, you can replace the heater core and re-assemble the way you took it apart.
Alcohol and and fowl language may be a temporary help.
HTH
Bob