It would have been semi-gloss... trim black is usually an acceptable gloss, but this probably depends on which brand. Are you planning on rattle can or gun spraying it? A rattle can lay out pretty even with the right technique. You could use a flat lacquer and apply a custom clear coat with the correct sheen if you wanted more durability.
Factory would have been semi-gloss black, no taped edges and a little bit of the overspray from the topside of the body colour bleeding around the bottom edges from being painted after... if you want that look you can probably replicate it, (but in in reverse order) with a little practice -it will be harder after the car is painted! Also depends how correct you want the car... most people (even the purists) will correct some of GM's awkward oversprays when they restore a car, even those that are diamond judged... for example exhaust manifolds would/could have orange overspray from the factory, so would have many aluminum intakes on engines with that option, or the water pump to intake hose on a bbc would have been oversprayed orange as well...many acceptable restorations clean up those details...

ak

Thanks guys! A pic would also be nice too see. I may try a rattle can, as the car is not perfect, but will look more origional with the underside of the hood the correct semi gloss black.

AK67SD nailed it

Thanks for the pic..I think i will paint it myself...as soon as it warms up a bit! Car was Butternut Yellow from GM, now Borello Red. Since i cant duplicate the red overspray onto the black, i will mask it along the edges....dont want black on the red!! Will send pics as it progresses...we all like to see pic's!!

Yea the underhood red matches the car, and is in pretty good shape! Just trying to get as origional looking as i can...would like to make it yellow again, but that would be too much!