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I would get a second opinion on whether it could be threaded, or not.
Contact your local frame builder.
Threaded and threadless steer tubes are not the same. You’re not supposed to cut threads into a threadless fork. The threads in a threaded fork are rolled pushing the threads out a bit.
It’s do-able, and I’ve done it but the three frame builders I’ve worked with refused to do it.
I would get a second opinion on whether it could be threaded, or not.
Contact your local frame builder.