DA, I'm not sure I know what "collored bushings" are but I wouldn't try to turn down "hardened linear rails" nor "thread them".
Could you not just grind a flat for setscrews in the tapped holes in the brackets?
Could you not just grind a flat for setscrews in the tapped holes in the brackets?
Busy Bee 12-36 lathe, Busy Bee Mill drill, Busy Bee 4x6 bandsaw, Homemade 9x17 bandsaw, Ad infinitum.