Check out obsolete Chevrolet: http://www.parts123.com/PartFrame.a...com&TITLE=Obsolete_Chevrolet_Part_Co

and Tucker: http://www.tuckersparts.com/AirConditioningHeating-tp2-81.html

Another option if you are unsucsessful finding the proper core you might be able to buy a complete heater from another year vehicle and jerry rig it under the dash.

~Jim