heater assembly interchangability?

The '66 heater box will physically bolt right in as long as the '66 hasn't got factory A/C. However, the '66 box is set up for control by two slider levers, one of which controls off-heat-defog and the other controls temperature. The '64 has three pull knobs: one controls temperature, one controls heat airflow and the other controls defog airflow. You may have to do some adaptation to make the '66 box work with the '64 control knobs.