I ordered the acme screw and bronze nut from McMaster and made the parts on my lathe. It was easier to create a new insert in the casting for the nut than it was to machine the nut down to a shape like in the drawing. The Y leadscrew works great now.
The Y movement isn't great because of the wear on the ways. I cleaned the gunk and lightly hit it with a whetstone to see if it would help (and it did a tiny bit), but it didn't do much. I talked to a local guy who estimated about $1500 to scrape it. I haven't contacted Shaffer Grinding yet.
I don't have it all the way together yet to measure how far off it is. It will have significant slop in the middle if adjust the gib for full Y travel, otherwise it will only have a few inches of travel if I adjust the gib for the wear.
I'm a beginner to this, so I don't feel there's real value in dropping $1500 on it now. OTOH, I imagine that a machine out of adjustment will make learning harder. I'll have to think about that, or live with it for now and fix it later.