I think either 91 or higher is fine, but based on the age of your vehicle I would suggest that you check that whichever grade you use does not have ethanol.
As I recall, Shell, Esso and Mobil's 91 does not have ethanol but Mobil's 93 does have it. So you need to do some research. Now if your car is a daily driver it may not be much of an issue but if you store it for any period of time then you do not want ethanol in the system.
I have an old 77 Oldsmobile with 403 motor (no mods, factory stock) that is supposed to run on regular ethanol fuel but pings like crazy with it. Run Shell or Esso 91 and it purrs. Just my thoughts based on my experience.