I'd say too much play in the hinge pivot pins. They get sloppy - actually can move around a lot, especially if they are originals and have cycled for 40 + years with minimal lubrication.
Unfortunately, the days of cheap GM hinges are over. I'm not sure of the repro quality - you could always have the originals rebuilt, or buy GM (and get ready with your wallet).
Make sure your hood side bumpers are in good shape as well - they should wedge the hood slightly when closed, keeping it centered.
A tip I learned a long time ago - leave your hood up when garaged, Keeps the springs compressed -
Regards,
Steve