New to timing?

[Lerch1145]

Have a 1972 GMC 350, just purchased a Petronix igniter, flame thrower coil, new cap, rotor, wires, and spark plugs. Plan to install tomorrow, set plugs at .045, and recheck timing.

Here is the question: when checking what my timing currently is (12) I was going to move down to 8-10 and see how she reacts. The manual states "adjust timing by loosening distributor clamp and rotating the distributor body" is this the clamp holding the distributor? I loosened this to the point of removal and still nothing moved.

What is a stupid guy missing here?