how do you set the timing on a 1998 chevy 3500 van

Simple way is old school. Remove the plug from front left cylinder, number 1 (drivers side) place finger or thumb over hole, and bump engine over until you feel air blow out, this is the compression stroke. Now use socket on crank bolt turn clockwise until the timing mark is lined up to TDC (0) Remove the distributor cap and be sure rotor is firing on number one plug. if not pull distributor and set back in on number one. one tooth does make a difference, be sure it's right. This sets your base timing the computer should make any further adjustments for you.