I have a very useful book you might want to get. The title is Pontiac Firebird restoration guide by Joe Moore. On page 16 it states that sway bars should be semi gloss black , but notes some bars were left natural. I painted mine detail gray because I like the natural look. I'd also suggest you watch a YouTube video "1967 under body markings by scared shiftless". They show the sway bar black and have great chassis shots. Hope this helps.
Doug