How do you set the timing on a 1994 Chevy truck?

If your truck is all original, AND has all of the under hood stickers, the answer is there. One critical point is to remember to disconnect the plug on the single black wire coming from the relay center on the vehicles left side of the fire wall. It's the "T" shaped plastic cover, usually black plastic.

Once you disconnect that wire, set the timing as specified on the label on the fan shroud. For instance, on my truck, the setting is "0". When you are finished, shut the engine off and reconnect that plug. Disconnecting it has something to do with the computer "seeing" the settings (I think).

For further information, refer to your manual, or a trusted dealership.