[vectorit]

I can't say for certain when it comes to 1972, but on my 1968 K20 the steering gear was left natural. So with mine when I cleaned it up, I left it cast iron gray and no paint.

Are you going for originality, or just aesthetics?
If you are not concerned with originality, I like the look of cast iron so I'd paint it with Eastwood exhaust manifold paint.

But... These boxes tend to weep oil, so black may be a better choice to avoid having to wipe oil off the gray paint periodically.