Pre-90 vert's are round 5.25"'s and are at the bottom of the quarter panel. I squeezed in 6.5's with minor modification.
90+ cars have the speakers in the middle of the quarter panel, allowing much larger speakers. Stock may have been 5x7, but if you watched the mounting depth im sure you could fit in 6x9's. Remember that when you buy speakers, mounting depth can get you.