Two years ago,I had to pull the evaporator on my 1979 Toyota Celica in order to change the pressure switch and expansion valve (both are contained in what Toyota calls the “Cooling Unit”.
Not too complicated: Remove glove compartment door (three screws), glove compartment, and you have all the room you need. Three bolts release the air duct next to the cooling unit, another three bolts for the unit itself. The hardest part was sizing a wrench to remove the low pressure hose that runs back to the compressor -29mm. Had to wait a week for Home Despot to get one in. Don’t know about the heater core. In 41 years, I never had a problem with it.