With regard to the timing light, or any timing method using a line clamp, it's not uncommon for readings to become erratic at higher RPM's. The technology works by sensing a capacitance change in the steel. This occurs because the line swells a little bit with each injection. It doesn't matter where on the line you put the clamp. The whole line swells, all at once. Unfortunately the changes are really small and vibration and other things can affect it. I've timed trucks where the timing is rock solid, and I've timed trucks where the reading jumps all around. It takes a practiced eye to tell the valid readings from the wrong ones under those conditions. Because the human eye is fallible where optical things that exist for only a moment can appear different then they really are (motion pictures are a great example), the timing light method is not the greatest when it comes to timing these trucks. A digital meter with the correct crankshaft probe and accurate readings is far easier to read under less than ideal conditions. My suggestion would be to time the truck by ear, and trust your gut more than your eyes when it comes to this kind of stuff. Remember that you can manually energize or de-energize the cold advance solenoid to obtain a 2 degree change in timing, so if you set it and think you would like to see what it is like with 2 more degrees of advance, just turn the cold solenoid on. If you would like to try 2 degrees retarded, simply time the truck with the cold advance on, and then disconnect it to retard. In this manner you can try 2 different timing settings without actually moving the pump. This will get you by until someone with a digital meter or practiced eye can look at it.
