Is your Mustang an export car? Domestic sold Mustangs in 1968, that had manual transmissions came with thermactor smog systems, or air pumps. I ask this because you said cast or natural for the alternator mounting bracket. 289/302 cars in 1968 with smog had a cast iron bracket for mounting the air pump (natural or cast grey color) and the alternator (alternator under the smog pump on passenger side).
If no smog and you have the stamped steel alternator mounting bracket, it would be painted satin black (chassis black?), same with the alternator adjusting arm. Alternator pulleys are not black, they are gold?
Color pictures will help you. I am getting ready to detail my 302-4V engine for my GT/CS very soon.
For power steering - aluminum bracket is left natural. Only the front bracket is painted black.
Pretty sure that the spacers for power steering (if without A/C) and the alternator are also natural.