Has A/C with automatic temperature control, have to clamp off a heater hose to shut off the heat
Under what circumstances is that vacuum operated valve supposed to close? Or is it a flapper door/control problem.

My money is on a flapper door. If the setup is like F-bodies (and I think it is), all incoming air goes through the A/C evaporator first, then either through the heater core or bypasses the heater core.

Better look under the dash and check those vacuum lines.

If its like my 84 d150 it only shuts the valve in the heater hose on Max A/C and "OFF". other wise it lets hot water pass. I determined this by putting a vacuum gauge on the line that connects to the valve and running it in all positions.

mod5v is correct. The water valve only closes on max A/C. If you have heat all the time, then the servo motor for the blend air door is bad. I replaced quite a few of these in my days at the C/P house. I converted my old 5th Ave to a cable from an old (late 70s'?) LeBaron from the wrecking yard. Cost almost nothing compared to a new servo, which was more than the 5th was worth . Hope this helps!

these cars also have a vaccuum fitting behind the controls that is prone to crumbling after 20 years....might be the culprit, too. of course, if that's the case, it should also default to just floor heat/cool