Ohh, that would be such a pain to do on the lathe!! Difficult fixturing. And no second chance if you mess it up and cut too deep or the work shifts in the chuck!

I would rather save the time and buy Phil Wood rings. You don't want to spend the money on a whole Phil Wood cartridge, so you just get the rings and use a Shimano UN300. It will work. They are the same diameter.

I have used the Sunlite threadless (came with a frame with stripped threads). It kept loosening up on me. I eventually welded and retapped.

Other things you can do include using the existing cup and cone system but putting a square taper cup and cone spindle in. They normally work with the Raleigh cups. Sheldon Brown has a thing about this on his site.