My custom toe arms are now anodized and assembled. I am posting the ball joint assembly details because they are indicative of how the M arm ball joints are assembled.
As noted previously, the cavity that houses the ball joint is hemi-spherical.
These internal components were salvaged from an M3 toe arm. Unlike some plastic ball joint bearings, this one is not split and it must be forced into position and fixtured there before the ball joint is permanently swaged.
Here you can see that the plastic bearing does not fully seat itself in the cavity unless it is forced into place.
This fixture presses on the plastic bearing directly to seat it fully. Pushing on the steel part of the ball joint is not effective in seating the plastic bearing.
The geometry of the lip and of the swaging tooling required a bit of trial and error to develop, but the result is a consistent and sound swage, with no sign of cracking.
And here is the big picture, showing the custom toe arms in comparison to the OE pressed steel part. The pin center-to-center distance of the custom arm is identical to that of the OE part.