99K is not a GM paint code on a Grand National or any other GM car or truck. The code is something like WA8555. 99K is a part number for Dupont black toner, black toner used in formulas to make other colors. Used by it's self it's "99K black". So you CAN NOT get a 99K PPG paint. They are bending the truth or description a bit if they are calling PPG a "99K" what they probably mean is simply the blackest black PPG toner. But it would have another number of course because it isn't Dupont.
It is not what GM used, not by my guess. They get paint from a few manufacturers I believe and it's probably a crap shoot as to what paint manufacturers paint would be on a particular car.
GM black is NOT very black, be it on a Grand National as far as I know I could be wrong but any black car or truck from GM is going to have the same code. It is MILES from "black" as I remember. That 99K wasn't very black either from what I remember. You need to get a little bit of each and brush it out on a piece of metal. Go to the paint store and ask for a sample spoon like at the ice cream shop.
As far as the value line "Shop line" I seriously think it would be as black as the high end stuff being from what I understand the blacker blacks require more grinding of the pigment. More grinding sounds like higher cost to me so the value line isn't going to have that.
Just get some samples and see what makes you happy.