Well, the part hasn’t yet arrived from Australia, despite the company shipping it out with all due speed.  Kudos to them for doing that, but the fact is it takes many days for something to get over here from Australia.  In the interim, I went at the suspect part with files and sandpaper and managed to get the ball nut to fit for now.  I will pull the machine apart and put in the real part when it arrives, as well as hopefully add in the optical limit/home sensors, etc.

Here is a bit of video with it running in free air, and then machining the default EMC2 G-Code into a piece of Corian.