With thanks again to Mr. Art Benstein and others, it would seem that I have successfully adjusted my heater valve so that it turns the heater completely off. The valve in my car is still the original with the "wax motor" temperature regulator. (Mine is a 1982 model 245.)

The symptom: I suspected that the heater was not shutting off completely as the fresh air coming into the cabin through the heater vents was warmer than that entering through the driver's footwell vent. (My wagon is old enough to still have such a vent, and a great feature it is!).

The solution: Following illustrated advice from Mr. Benstein, I adjusted the wax motor adjustment screw at the heater valve and now the heater shuts completely off. Less unwanted heat for my revived AC to overcome! On my car, the screw was a Phillips head, and I was able to wiggle the valve sideways and reach it with a screwdriver. I went two full turns clockwise, but I think a more fair adjustment could be made once it's not so blistering hot.

For more details, see this earlier post: http://www.brickboard.com/RWD/index.htm?id=986870

Thanks again for all the help!