Well, as mentioned no cab corners are perfect, but at this point i wouldn't even call most suppliers acceptable. Since they are cheap i got 4 different kinds of cab corners just to experiment with and i am now resorting to buying a used piece for the fit I like for the body line 1/4 way up from the bottom of the door/corner and using after-market metal for the very bottom. Because they are cheap i suppose quality suffers too. The best advice is to limit the amount you remove. Im restoring a 69 Firebird with my brother as well and parts quality is much better.
example of one of many issues:
