Switch location update
Yesterday, using a stack of towels and 2 foam pads, I was able to crawl on top of my engine to see that the sending unit is where Ed (and others) said it would be, adjacent to the distributor, basically beneath the vacuum advance, down below the level of the intake manifold, difficult to see under all circumstances. I used a miniature Rigid pipe wrench to try to remove it, and it wouldn't budge, despite my best efforts. I really wasn't able to get much leverage on it at all. Every position was clumsy.
It appears that the best way to remove it would involve pulling the distributor and the steel engine vent tubing. The sending unit does not have flats on it, but is more rounded, with little vertical slots around it, perhaps to accommodate a special socket tool.
It's not a pressing project. I may do more work on it before PCS Hudson, but it has moved way down the list, since I have lots of chrome to polish. Thank you, everyone, for all your help! Tom