The ignition advance in this truck is controlled by the ECM ( Engine control computer), in order to set an accurate ignition timing it is necesary to by-pass the ECM, to do so, you need to remove the in-line spout connector located on the left inner fender well near the Ignition module.
To remove the in-line spout connector, pull gently on it with one hand as you hold the electrical connector with the other hand.
The harmonic balancer in this truck has the timing marks stamped on the housing, from TDC ( top dead center) 0-10 degrees retarded and 0-30 degrees advanced.
If your timing light doesn't have adjustable advance feature, mark the harmonic balancer housing on the 10 degrees before TDC, ( 10 degrees BTDC is the initial timing for this truck).
Next, loosen the bolt that secures the distributor hold down clamp in place, the bolt needs to be loose enough to turn the distributor by hand, but not so loose that it turns on its own when the engine is running.