The heater core is removable from under the right side of the instrument panel.
See Figures 1 through 9
Plug the heater core tubes to avoid spilling coolant in the passenger compartment during removal.
On some early model vehicles, the cabin light mounted under the dash may interfere with cover removal. If necessary, loosen the retaining bolt and reposition the light housing with the wiring attached.
On a few of the early model vehicles covered by this manual, some of the heater core cover screws may be partially/fully covered by the dashboard. Be sure that all are removed before attempting to remove the cover. If necessary, unbolt and reposition all/part of the dashboard for better access.