I have the tranny pan that is a rectangle with a corner cut off, so I am assuming it is a TH350 tranny. Also, in an '81. Anyway, my question is where is the modulator valve? The shop manual says it is on the LEFT rear of the transmission, but the only thing I see that looks like a vacuum can is on the RIGHT rear. Also, the hose going into it does not look like most vacuum lines. It is harder and appears to be thinner walled. Am I looking at the modulator valve? I am trying to do a Ken Styer trick of removing the hose and tightening a screw inside the hose fitting in order to firm up shifting...

Yes, that is the Modulator Valve, right side rear. Small hard line with short connecting hoses. Don't know about the adjustment though.

You found it. Remove the hose and reach in with a pocket screwdriver. Straight slot, not phillips.
Turning in will give a higher shift point. Out will lower it.