You didn't mention the series, but assuming it's a 40, there are a few things that could be happening.
Pull the plastic plenumn that runs from the blower motor box under the glove box to the heater box. You can now see the squirrel cage for the blower motor. Turn it on and verify that it's blowing at the various speeds. Now look in the heater box at the top of the heater core. They often get plugged up with mung. It's not too hard to pull the whole assembly, but if you don't want to mess with draining the coolant, you can clean it best you can through the hole.
The other big thing is the seals on the heater doors and the doors themselves not moving correctly. Check to see where air might be leaking when the little flaps are where they are supposed to be when they divert air to where you want it to go.
Also the heater duct tubes that go from the box to the defrost ducts fall off and get loose. There are ne ones available or you can replace them with other flexable tubing.
Lastly, the heater systems on 40s generally suck anyway. You can replace it with a JC Whitney or Vintage AIr unit for better perfomance.