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.
Next, connect the positive and negative wires for your timing light directly to the truck's battery.
Next, connect the spark plug wire adapter to the number one spark plug wire, ( passenger side front), this is how the timing light gets the signal.
If your timing light has ignition advance feature like the one in this picture, set the dial to 10 degrees, the 0 mark on the harmonic balancer needs to be aligned with the timing mark with the engine running, make sure that the timing light wires are away from the engine cooling fan and accessory belts, otherwise they will get cought on these moving parts.
The ignition rotor inside this distributor in this engine turns counterclockwise, to advance the timing you need to turn the distributor clockwise, to retard the timing you need to turn it counterclockwise.
Be very careful when you are moving the distributor, don't wear loose clothing, you can suffer severe injuries if your clothes get cought by the accessory belts or engine cooling fan, if you have to turn the engine off several times as you turn the distributor, that's ok, it is better to spend few more minutes on this process in order to be safe.
Once you are done setting the initial timing advance, remove the timing light, tighten the distributor hold down clamp, install the in-line spout back on.
It is possible that the check engine light may come on when you remove the in-line spout connector, to clear the code without a scanner, unplug the negative battery cable for 10 seconds.
Test drive the truck, make sure that you don't hear any pinging noises caused by over- advanced ignition timing, sometimes the harmonic balancer shifts as it ages causing innacurate readings.