Even if you know that the flywheel you have is the right one for your engine, setting timing to the ball lining up with the pointer usually only provides a rough timing setting - good enough to get it started (either by static timing, or by using a timing light).
I have found the most reliable "final" setting (on a 216/235/261) is obtained with a vacuum gauge (maximum vacuum at idle on a warmed-up engine), or by setting close to the engine pinging under load. Adjust the timing to the engine, not to a setting (and, it also saves having to buy a 6v/12v timing light).
The bonus of having/using a vacuum gauge on these old engines, it the vacuum gauge is also useful for setting carburetor idle mixture (for one carb, or for multiple carbs).
