Mark,
I use Mach3 version 062,with a parallel port, and threading works just fine. I have the G76 set so that it posts G32 with comments.
I like to monitor the threading cycles as they are being done.
See page 47 in Threading On The Lathe - Mach3 Turn on how to modify the Macro M1076 if you desire to G32 output of the code.
Not sure what your lathe setup is and that may be your problem and why they recommended what they did.
Not sure what Fusion post processor you are using. You can download one called mach3 turning.cps from the Autodesk site.
Here is a link :
http://cam.autodesk.com/posts/?p=mach3_turningI don't want to want to post the modified post processor since it's modified to work with the way I work and you may desire some of the posting that the generic one posts.
Your posted code runs fine here and note that the G28 ( see attached )will cut through the material.
RICH