I don't know for certain that's why the change was made. Not mentioned in any Chevrolet service bulletin. But it makes sense. Considering the variables involved-air cleaner stamping, carb gasket thickness, element height, depth of insertion of the vent tubes, it may have been possible. Appears to have been intermittently phased in. I know of a Sept '69 Z/28 with a revised base and an October '69 Z/28 with a flat base.
The 90º vent tube may have never changed in production. It was only a problem on Holley DP carbs. ZL1s were supposed to be built with the 850 DP but most were not.