Removing the heater core - not a big problem. Remove the RH lower dash panel (stainless steel strip, three screws, two bolts). Then the heater housing cover (7 screws as I recall,all with 5/16"hex head). Move the bowden cable for the RH air vent out of the way. If the heater core has been removed before, there may only be 2 screws to remove to loosen the core from the housing. On the engine side of the fire wall, remove the heater hoses and 2 tin nuts above and below the hose connections (7/16"). Core comes out!
But, if the heater core has 4 screws holding it to the housing, then you need to remove the other nuts on the engine side to remove the heater housing. Been there, done that!!