I noticed my IROC has 1/2" studs and lugnuts on it. Did someone modify my car or is that stock for IROCs? Mine is an '89

Had to be modified. All third gens come with 12mm x 1.5 wheel studs.

Ohhhhhh
Maybe the 1/2 20 nuts were just forced on the stock studs....

That doesn't sound safe

I doubt it. 12mm is closer to 7/16" than 1/2" and the thread pitch isn't the same.