1976 Dart Overheating and other issues

From the look of the thermostat housing, the radiator is certainly plugged. The water pump impeller may also be gummed up.
I would do a general overhaul on the complete cooling system:
New or recored radiator
New thermostat
New-replacement thermostat housing
New upper-lower-bypas-heater hoses
New water pump
If you plan to use the heater, new heater core