Does the center hash mark on the gauge at the base of the dizzy represent 0* or theoretical TDC of #1 piston (assuming the engine is timed with the marble embedded in the flywheel)?
My info is based on a GMC engine, but I'm pretty sure they are the same as your Chevy.
TDC on the flywheel is the vertical line between the U|C.
The "ball" is where the manufacturer wants timing to be set at a very low RPM with the vacuum advance disabled.
Loosen the clamp bolt that secures the plate with the "center hash mark", set the hash mark to zero and tighten the clamp bolt.
In the case of my GMC, the ball is 5 degrees before TDC. GMC says to reduce engine idle speed to 350 RPM when setting timing because mechanical (centrifugal) advance begins at 400 RPM. Loosen the clamp bolt that holds the vacuum advance to the distributor and use a timing light to set timing to the ball. When timing is set, tighten the clamp bolt and check that timing is still correct.
Once timing is set, adjust idle speed back to spec and re-connect the vacuum advance.
I have found that my engine performs better with an additional 4 degrees of advance. By using the octane selector (GMC calls it the "Manual Advance") I can dial in a few extra degrees of advance without the use of a timing light. Just loosen the bolt, move the mark towards the "A" and retighten the bolt. The hash marks are actually about 1.5 degrees apart. If you don't like the way the truck runs at the new setting you can always move it back to the zero mark without the need for a timing light and timing will be back at factory.
