The threads on the end of the original cross slide shaft are badly damaged. They could not be repaired with a 12-24 die. The plan was to remove the damaged end at the line labelled 'cut' in the diagram below and replace it.
It is not clear what the what the length of the 12-24 threaded end was originally.
The first job is to make an acceptable replica of the end of the shaft as sketched above. The part must be about 0.50" + 0.10" + 0.3285" + 0.070" + 0.25" = 1.2485". To maintain the most accurate concentricity the part will be turned between centers. Two 90° corners are probably needed on the left or cutoff line end: where the 1/4-20 screw joins the 0.27" diameter and where the 0.27 diameter joins the 0.31" diameter.
The plan was to face off to length the 12-24 threaded end once the length was determined.
A 1.3" piece of 3/8" drill rod was cut with a hacksaw. Both ends were faced and center drilled. The shaft was placed between centers and then the diameter was reduced to 0.312". On one end the diameter was further reduced to 0.215" for a length of 0.320". Thread relief was cut from 0.250" to 0.320" to a depth of 0.163". The shaft was flipped between centers and the right end was reduced to 0.2715" for 0.60". The first 0.500" was further reduced to 0.250". Thread relief was cut to a diameter of 0.200" and for a length of 0.040". The long end was threaded 1/4-20 and the short end 12-24.
A new tailstock center was required for this work as the old one could not hold the tolerance. When making this tailstock center the headstock offset angle was set to 30° with the dial indicator. It was then heat treated and quenched in oil.