katrina,
What would be the GM part number for that tool? My guess is that it would have been a Kent-Moore tool, if it was a GM tool? I see no special tool mentioned in Chevrolet truck or GMC manuals. I have seen a
non-GM Zim-Zip valve adjusting tool that sounds like the tool your describe. That tool could be used with any valve adjustment technique (and makes it easier).
I have seen Chevrolet Truck Shop manuals give specific warnings about how to make sure that the engine temperature is warm and stabilized. Before adjusting valves using either technique, the engine must run until temperature is properly stabilized. And, the Chevrolet Truck Shop Manuals state that after reaching a stable warm temperature, the valve cover may then be removed and the valve stems should then be lubricated.
I have not read a specific reference in a Chevrolet Truck Shop manual that the valves should be adjusted with the engine running or the engine not running.
I can easily determine where a knock is and where a loose valve is, by using a cheap stethoscope with the engine running. Then, that valve could be adjusted tighter with/or/without the engine running. Of course, I would have first made sure there are no grooves in the face of the rocker arms - this will also allow feeler gauges to last longer.
I have no old GMC manuals but I have a 47-54 manual. Usually, the GMC manuals have clearer descriptions of techniques - in the case of adjusting valve lash I cannot find as much information as stated in the Chevrolet manuals.
