How do you adjust timing for a 1984 GMC 3500 vandura?

To adjust the timing on a 1984 GMC 3500 Vandura, first ensure the engine is at operating temperature and set the idle speed. Use a timing light and connect it to the number one cylinder's spark plug wire. Locate the timing marks on the harmonic balancer and the timing tab, and loosen the distributor hold-down bolt. Rotate the distributor slightly until the timing mark aligns with the specified degree on the timing tab, then tighten the distributor bolt and recheck the timing.