See Figure 1
Timing should be checked at each tune-up and any time the HEI system is serviced although it isn't likely to change much. The timing marks consist of a notch on the rim of the crankshaft pulley or vibration damper and a graduated scale attached to the engine front (timing) cover. A inductive pickup timing light must be used, as a neon(static light) is too inaccurate. The inductive pickup clamps around the No. 1 spark plug wire, sensing the surges of high voltage electricity as they are sent to the plug. The advantage of this type of light is that no mechanical connection is inserted between the wire and the plug, which eliminates false signals to the timing light.
Some timing lights have other features built into them, such as dwell meters or tachometers. These are convenient, in that they reduce the tangle of wires under the hood when you're working, but may duplicate the functions of tools you already have.