HI Chris, I agree with John H as far as getting the cart before the horse !! however it still may be possible with careful setting in a 4 -jaw, difficult to advise fully without seeing the job.
As for the tool since its a one off job would grind one from HSS and use it in a substantial boring bar, assuming there is a shoulder inside the work I would arrange to start inside with an undercut and screw-cut outwards L -R just make sure you are cutting a RH thread !
Because of the size I would also arrange things so as the thread gets deeper you can feed in say 2 thou on the cross slide and 1 thou on the top slide thus reducing the cutting load on the tool.
Might be wise to make a male gauge first to test the work.
John