Greg,
does your heater motor stick through the firewall? If so, I think that it may be the older 48 style that I have only seen once. If your heater is the standard type "recirculating" heater, there is a thread on "OldGMCTrucks" that gives the name of a place in the northeast that has the cores. They are expensive though. My heater in my Canadian truck says GM on the logal and it is a recirculating heater.
