There was also the matter of getting the pipes to line up, and then there was the wall... Things were nailed in with boards and chunks of 2-by-4, which first had to be removed even to get at the old fixture. Then he had to cut a bit deeper into a (super-hard heart redwood) support beam (but not so deep as to weaken it), and put wedges in behind to fill what he'd taken out (so it didn't weaken the bearing capacity too much), and then he had to lever the pipes into place and secure them with brackets, to get them far enough through all the layers of wall to be able to attach the extenders (which had to be called in and picked up/delivered by his boss, since there's only one place in San Jose that carries them). It seems that the length of the valve housings on current plumbing (the part that extends through the wall, that the handles attach to) is about 1 1/2 inches shorter than in the 1930's, and they don't make the longer version any more. (After all, nowadays, nobody has thick shower walls, right?) The plumber thinks it's part of the conspiracy of planned obsolescence, and making people completely remodel, rather than simply replacing parts. After dealing with vintage plumbing, and finding out how they seem to reinvent the standards every 20 years or so, I can't say I disagree with him.
So I now have an upgraded tub/shower, with new pipes and modern valves and handles, and it doesn't leak and actually works. (The hot water has been a little if-y from the time I moved into the house fifteen years ago, and has only gotten worse and worse over the years.) I need to replace the drywall, now, but the important thing, the actual plumbing, is redone!