Update on the nut and clip I was searching for on the air vent handle/knob. I went to see a friend today who has an original 68 GT 390 and also a bunch of original parts. He happed to have everything I needed and I plan to go back again for more original parts.
Here are the parts in question. Also showing the differences between the 67 version vs the 68 original and repop versions.
Here is what the clip looks like for the 68 air vent handle/knob
68 Reproduction on top, then 68 original, then 67 original.
67 style nut