Here is my version of the fronthub for the Tamiya Porsche 959 (58059) and Toyota Celica.
The original has the balls molded in. I do not have any 6mm ball joints to take measurement of. But the kingpin hole is set to take M3 tread. The balls center distance should be about 27mm, the hub is 19mm.
This version is not reinforced, but as close to the original as i could measure.
I found a supplier who sells 6mm balls:
http://modelshop.co.uk/Shop/Item/M3-tapped-stainless-steel-balls/ITM7332
Use M3x10 setscrews to mount them to the hub. Shim out accordingly.