All you need to do is check the diameter of a G70-15 tire that came on the '69 A12's because they had to add a .250" shim under the bump stop to prevent it hitting "something" by the engineering drawings/specs.........that doesn't answer how wide the tire can be (if that's what you're asking) but as far as "what's the tallest tire you can put on a '68-'70 B-body to engineering concerns then a G70-15 or maybe a 15" 78-series tire should give you a ballpark answer.
