older cars draw all of the air for the heater from a plenum at the base of the windshield.. all of that stuff was built into the HVAC "suitcase" on the G bodies starting in 78, so if your entire HVAC sytem is gone, then the air intake is gone.. you could do some fabrication work to get air from the base of the windshield, or you could find a complete heater setup from a non-AC equipped G body and put it in your car.. but good luck with that- they were rare when they were new, and there aren't very many left...