Make sure there aren't any pin holes in the cooling system. That will keep it from making any heat as well. I had a 68 Plymouth Valiant with a 225 in it and a small pin hole in the radiator. It never really leaked just seeped a little. That was enough to keep the cooling system from building any pressure and kept me from having heat. One winter here it stayed in the 20's for days. Driving to say the least was miserable. I even took the fan off and put cardboard in front of the radiator. At traffic lights the needle would just barely move and you would start to feel heat. Then you started moving again and cold air coming from the heater. Have your cooling system pressure tested as well to make sure there aren't any small leaks.
