The lever in the heater core box (under hood) is for the deluxe heater and diverts the cowl vent air thru the heater core for hot air or around the heater core for cool air.
The thrift air is the controls you describe on your dash and the lever (heat/defrost) on the diverter box (not sure that is the proper name) under the dash. This control is a manual adjustment, not controlled by a lever on the dash.
If you choose to make the whole thing a deluxe heater set up you will need to get both the diverter box and control levers.
The control levers are available aftermarket, haven't seen the diverter box in a catalog so this is most likely something that you would need to search for at a boneyard.
