Speaking from experience in the school of learning the hard way.....

20570R15 is the best size to fit any A body 67-72,even with 73-up disc brake conversion.
Rim offset will determine if the tire would hit the fender bolt back of fender and front tip of fender.
4" OR 4.25" offset is best,3.75 be carefull.
21570R15 67-72 will catch the top of the fender with Cragar S/S wheels (3.75" offset) and Ralleye wheels (4" offset).
73-76 A body 2157015 works great with factory disc brakes.

From what I can remember,the 73-up brake set up is wider overall compared to the drum and K-H disc set up. So when the 67-72 cars convert to the 73-up brakes the wheels move out closer to the fenders. Keep this in mind when going with larger wheels and tires.
Hope this helps