Alright, I replaced the vacuum modulator. No leaks and all seems to be installed right. Thanks to Mike D for helping me identify it. The old one was definitely trashed. You could see the spring inside had broken and was visible from the side where you should see the allen bolt only. I don't actually know if there is still an allen bolt in it as the spring is all jamming the orifice so you can't see.

It didn't change shifting much. Still no 4th gear and still a little flaring between 2-3. Not as much as before, and it runs stronger as well. I adjusted the allen bolt 3/4 turn clockwise before it started feeling hard to turn.

I stopped as I had two conflicting recollections of the mechanic I purchased the vacuum modulator from telling me to turn it 1 or 2 complete revolutions to adjust, and someone else telling me to turn it 1/4 turn at a time. I suspect it may have something to do with different styles of vacuum modulator. I have the old style (no plastic). Anyway, I'm looking for clarification before I go crank it too hard.

Anyone know if you can crank it too hard? Can I puncture something in there?

I did things a little backward as most recommendations suggest
checking vacuum lines first. I figured that I would replace the vacuum modulator immediately as it was definitely leaking. So now I'm off to the vacuum lines. Hopefully Monday. I'm going to start at what I think is the vacuum manifold. It's just behind the carbs and it seems the line going to the transmission is metal.

Anyone have a better diagram than the one at mzf bayern? That one seems to be missing a few parts and labels.

BTW, MikeD, I am beginning to think your skepticism of the mileage is warranted. I found an extra speedometer? cable going from a metal box on the left front fender with vaccum lines going into it. There is a newer looking speedometer cable going down the the transmission which records apparently accurate speed. Could this vacuum/speedometer box have something to do with shifting?

Mbbuff, thanks for your comment. I've got to have a 4th gear in there somewhere.