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What other people have already said, but also: you can use the front brake on the back and just use a nut. Then you drill out the rear hole of the fork so it's wide enough to take a recessed nut, and buy a longer recessed nut that reaches the threads of the shorter bolt (cos you're now using a rear brake on the front which won't be long enough on its own). I've done this a few times now and it works a treat.
Also sawing a seatpost out of another frame, fun. In other projects - has anyone widened a brake bridge hole in an old steel frame to fit modern calipers? Is that the simplest/best solution?