99% of my restoration bolts are GM.
There is one nut particular to the G-body that is HARD TO FIND but it's kind of highly visible. And that's the master cylinder to booster nuts. They're flanged nylocks without much fanfare, but it's GREEN nylon. Most of the nut and bolt houses, and even GM, sells the brown or black or some other color nylon. But NOT the factory green. And I've never seen one of those come loose.
They look a lot like this, only with green nylon. Check them at a car show next time. GM replacements are just crimp style flange nuts. They're perfectly fine too, but not factory-supplied.