Panzer, I decided to just double-check my gearbox adjustment.
Several years ago I resealed the box, and I rigged a little lever/weight setup just like the book shows, to set the friction load on the workbench.
It all went well, and the steering wheel felt good, but the car still wandered.

But, yesterday, I loosened the jam nut, and was able to turn the adjuster screw 1/4 turn CCW before I felt any resistance at all.
I tightened it down right at that point.
Lo and behold: the wandering seems to have diminished noticeably. There was a pretty good wind yesterday, and we drove about 20 miles. Still moves around more than I think it should, but feels better.

DG