Not had to do one of these but over the years I've rebuilt quite a few ball nuts for people on here, on some of the more expensive assemblies the balls themselves can be of different sizes, this is to do with preload etc

Not done any spindle grinding personally but have seen it done this way on the tube, think it was an Abom video...

Pleased you kept the enclosure and went for a restore, would have been a shame to lose it I think, look forward to seeing her alive again, good luck and keep going