I hope this solution doesn't offend you .
Take two bolts, chuck on in you lathe and the other in the tailstock.
Thread the nut on .
Advance the tailstock towards the chuck.
Align the bolts and threads so the nut can screw smoothly between the two bolts.
Tack weld the bolts togeter.
Remove bolts from lathe and finish welding.
Return to lathe and re-cut threads.