A quickie update
Here's the REAL frame alignment tool ($17 at the local hardware store, who knew they carried these things?

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I didn't want to drop money on tapered bushings, so I went this route instead. The headtube bearing races have radiused corners and the fit is very good (well, not to a machinist!) so this should work out for my first attempt. It's 5/8" steel threaded rod, grade 8 washers, and grade cheap (2?) nuts.
I may go buy some more nuts so I can lock them down before I start the welding, which wont be for a while!