I'd say that at some point in the past (before you got the truck) the cap was put on cockeyed with the rotor missing the inner electrodes and cutting into the cap a bit, likely no harm done as it did not start and was quickly corrected. However, the center carbon button was riding on the edge of the rotor electrode and got chewed up enough to start a bad wear pattern. Over time your center electrode eroded down and started arcing a bit, but your miss was due to the plug lead being loose, which coincidentally aligned with the cut in the cap. Those jets you adjusted on the fly only affect the idle circuit, up to about 900rpm or so. The easiest way to adjust them is with a vacuum gauge, setting them for the best vacuum at your selected rpm. Like I said before, I'd replace the cap and rotor and see if you can't forget about the distributor internals once you get your distributor clocked right with the cam to allow adjustment.