personally- i wouldnt waste my time now with the old parallel port- it was done as a hack by art to get mach 3 going- (and worked darned well for most things- router and mill based)
if you want to do threading properly- add a cheap(ish ) motion controller- where the motion is calculated real time in hardware- not software. hit and miss/ will it wont it scenario isnt much good when your making a part- especially if the material and tooling is expensive.
ken crouch was one of the first to do real encoder threading using a galil controller. ive tested the code here for him at the time.
the results are superb- even being able to stop and restart a thread half way along. - skip to 2.3 of this video to see how easy it is.
https://www.youtube.com/watch?v=JfnQxDPWoEQ