jim rozen wrote:

Another approach is to leave the threading to the last, and then
bore the backside to accept a cylindrical bronze nut that is
pinnned in place. This makes the ram threads easily replacable,
and it separates the fabrication job into smaller bites.

That is a very good suggestion. My Smithy tailstock was made that way
with a threaded cast iron bushing locked into the end of the ram. When
the threaded bushing wore out I made a new one from bronze. Details may
be seen at
http://www.metalworking.com/dropbox/_2001_retired_files/TAILSTOK.TXT
and associate drawing and photo.

Although my problem was different than the OP's, the conversion would
make the job easier.

Also, if the OP has a boring head, it could be mounted in the lathe
chuck. The tailstock could be positioned such that it could be moved
with the longitudinal feed and the tailstock could be bored on the
lathe. This should make for excellent alignment.

Ted