Thanks for the help everyone. ESpecially FBinNY, you're always such a solid source of information. I finally finished the project! After calling nearly all the local shops and a couple framebuilders, it turned out the LBS physically closest to me was willing to thread the fork for $30. They did a little bit of a rough job, but it was good enough to set the headset up properly.

This project was such a mess! Definitely a good learning experience for me to check compatibility of every variable. Because nearly everything that could be incompatible was.

-Steerer length: too long! Bought one threaded fork that was just 7mm to short. Had to custom thread a threadless fork (or use a long threadless fork and a new headset/stem).
-Headset thread pitch: original was 26TPI Raleigh. Had to get a new headset for standard 24tpi.
-Wheel size/clearence: New fork did not have clearence for the 27" 1-1/4" tire that was originally on the bike. I gave my friend a 700c wheel and a 23mm tire. Thankfully they were spares I happened to have. Bonus points on the better braking on the aluminum brake track.
-Brake reach: The original fork was designed for standard reach centerpulls. I had to give my friend a spare sidepull I had in my parts bin. Bonus is that the between the dual pivot, kool stop pads, and aluminum rims, the front braking is infinitely better.
-Crown race diameter: New fork is 26.4mm, and I could only find complete 27mm crown race headsets at the coop, so I had to buy a new headset.

At least the head tube ID was normal?

I also donated way too many parts, and I'm going to eat the cost of a lot of this because I didn't consult my friend about everything--I was just determined to get the bike back on the road. The welded fork looks a little funny on the lugged frame, as does the modern pseudo-aero black rim, and black spokes, but frankenbikes are clearly the new hotness. On the plus side, the bike is now meaningfully lighter, and will stop a whole lot better, particularly in the rain. And I got a lot of experience out of it.