Yesterday I took it out on its "maiden" voyage.
The verdict? It's close, just a tweak here and there, new tires, and it'll be finished.
Here's how it looks now.
There was one more addition to the parts that I listed in the previous post: a front derailleur. I found that in dropping the chainring size to 46T (a common cyclocross size), and bringing the front derailleur down the seat tube to get proper shifting, the long cage of the triple rested on the chainstay. Hm...
I just happened to have an old 7-speed Shimano 600 front derailleur in the parts bin. Works beautifully.
So how was is the ride? Sprightly, responsive, as a good steel frame should be, faster than many of my recent rides over the same course (could be the weather too, as we had nice sunshine and warm temperatures). The braking performance isn't all that great, but that's what single pivot brakes do, so no surprises there.
After I got home I saw that the old tires were flaking off dried rubber from the gumwalls. I'd planned to replace them before shipping the bike anyway, I'll just have to do it before I ride it again.