How do you replace the heater core in a 1978 El Camino?

To replace the heater core in a 1978 El Camino, start by disconnecting the battery and draining the coolant from the radiator. Remove the dashboard by taking out necessary trim panels and bolts to access the heater box. Disconnect the heater hoses from the core, remove the heater box, and extract the old heater core. Install the new core, reassemble the heater box, and reconnect everything, ensuring there are no leaks before refilling the coolant and reconnecting the battery.