Thanks for your input, I don't mind trying though.To illustrate the point I am making get a bolt and run two nuts down it. Bring the two nuts together you will see that you can either have them locked together, in which case the bolt won't turn, or if they are sufficiently loose to allow the bolt to turn between both at the same time, then they aren't locked together. You can't have it both ways.
I doubt I would have the ability to make up another solution like on the lathe.
Seems as your saying the original hole would need to be drilled out and no threading involved,
but I still think there's a chance of achieving this, as there maybe wear of those threads or whatever
to allow some clearance.
This in my eyes is not the same as two nuts in which the flats wouldn't match as I'm hoping to get the fit for the quill with no play, and tap, then fettle afterwards.
This is evident on the return spring split thread, where this is achievable, agreed it's not the same thing either, but is a good illustration of the tolerance/slop required
Agreed might need to clean the bottom of the slot to some extent, and the sides will require some time to achieve no slop nor catches.
I've got the files and plenty of markers to scrape some witness marks down a bit.
Not decided on what material the gib will be yet, might try a ply from some hinges yet.