It's still awfully cold

[Elvis]

Frustrating isn't it? I'm not as knowledgible about these things as some of the others but it is not really rocket science either. I've had some of the same issues with my '67. Firstly, I also had a problem with the coolant being burped out of the overflow. This would happen when the thermostat opened. I did all the things you are being told - cleaned out the radiator, replaced the heater core, etc but it still did the same thing. I had a 195 thermostat but I think it was opening too late causing the coolant to boil out when the thermostat finally opened. I replaced it with a 160 thermostat and it works great. Runs a bit cooler but thats OK with me.
Regarding the heat issue, here's my understanding of it, and once again, I'm a rank amateur. If the water control valve opens as it should the hot water should flow to the heater core allowing it to radiate a lot of heat. If the correct plenum doors open, the fan should blow air over the heater core and then out the heat/defrost vents. In my view, if that is not happening, either the heater core is not getting hot or the correct doors are not opening. Sorry if this is very simplistic but that is a sign of my simple mind. E.