72 Slant Six running too cold

About the temp gauge on the cap... It isn't reading the temp of the water in the engine block. Plus if the hood remains up so you can see it , you'll wait forever for the engine to heat up. I recall a fellow worker diagnosing a VW Rabbit. So long as the hood was up the engine would run all day long and the electric fan never starts. If the new thermostat isn't installed backward the engine water in the engine will hit close to 180 degrees. I suspect yout problem is the sender or the gauge.
As far as the heat in the car, several issues effect that. Blend doors not functioning properly, heater cores' air flow block with debris, etc...
Solve the water temp issue first them look at the cabin heat. Good luck