You might have something plugging or even a bad water pump
You have a service manual? I'm guessing no. The latest we have for (free) download is the 73 which should be the same, here at the end of this thread
http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617
More info available from MyMopar
http://www.mymopar.com/index.php?pid=31
If this was originally a NON A/C car, there is no vacuum heater valve. You might have something like a mechanical door sticking. You might be able to open the vent door and look in there and into the discharge chute with a mirror and flashlight and see what the doors are doing with cable movement
Are you getting good airflow with the blower on high? Does the blower "sound" OK? IE run quiet?
Check your heater hoses. They should both be staying HOT with the engine warmed up