Traded the ancient Continental tires on my no flare 74 today for some Bridgestone Potenzas. I noticed that the contour of the two tires is much different... the Continentals are squared off and the Potenzas almost make a semicircle.
This reminded me how important your choice of tire brand can be. On my car the Continentals came very close to rubbing about 2" above the outside edge of the rear fenders at the top. The rounder profile Potenza has given me another 1/2" of clearance just where I need it.
So remember to think about profile as well as size when trying to squeeze the largest possible tire under the fender.