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The distributor body opening for the timing lever should limit the spark advance to 20 degrees. A 20-degree limit was appropriate back when the Model A was and gasoline octane was about 50. However, today we have 87 Octane gasoline, so the engine can run with more than 20 degrees maximum advance.
Some folks time the engine with the spark lever down a few notches because they know from experience, it makes their Model A's run well.
I suggest that you use the distributor body with the widest opening that permits the greatest amount of full advance, and iterate the timing by adding lever-down notches until you find the maximum advance that your engine will tolerate.