I'm going to take a wild guess and say that you've got one of the earlier JH's that don't have the heater vacuum switch or you do have one and it's broken. In addition, it sounds like the heater vents are semi-open all the time. Most likely because the adding that seals them is worn out.
Now, the nice thing is that this is all fixable. Check under the hood for the heater valve to see if you have one and get back to us.
Also, worst case scenario you can undo the heater pipes and insert a bypass so that coolant doesn't go througthe heater core.
It's a 1974 1/2 - don't know if that's what you mean by "earlier JH's" or not. The rest of your suggestions I will (hopefully) check out this weekend. Thanks, Dan