74 Duster heater valve vacuum or mechanical??

Yes, that describes my panel. (I assume you are referring to the horizontal fan switch?) It's quite possible I have different year heater box and control panel.

But there are most definitely pins and clamps for three control cables as I described, and only two cables on the heater box (one fresh air door which I thought was called the "blend" door, and one door to select between defrost and floor vents).

The pin on the temp lever moves the fresh air door to make the air hotter or colder, as expected. One pin on the off/heat/defrost lever moves the heat direction door from floor to defrost when the lever is moved between HEAT and DEFROST. So far so good.

The remaining pin, also on the off/heat/defrost lever, only moves when the lever is moved to OFF, so it has to be for a shutoff valve...

With no a/c, headers and maybe no interior, I don't want any more heat than I'll be getting anyway, so I definitely want the core off.

I got the 74827 valve today and now it's just a matter of finding a suitable cable and cutting the 5/8" hose to install the valve where the cable ends.