The truck has an Engine Control Module(ECM) that controls the engine. As long as you set the timing correctly, it doesn't matter what degree the timing is advanced or retarded, it's computer-controlled from there on out.
To set the timing, disconnect the ignition spark advance (brown wire with black stripe) wire. The wire is located under the carpet on the passenger side floor under the dash.
Set the timing and lock down the distributor.
Re-connect the spark advance wire and the timing will be controlled once again by the ECM.
When you reconnected the wire the computer took over and advanced the timing to as much as it could handle based on all the sensor inputs. Your computer constantly tries to give you the maximum timing the engine will handle safely. By readjusting the base setting you are actually hurting yourself because when the engine is in open loop (not paying attention to the O2 sensors for various reasons) it uses predetermined settings from a table of values. Trying to advance it more just makes those values wrong and if the knock sensor kicks in the ECM will pull back the timing a bunch.