George, my measurements are a close match yours. This is on a 1988 stock C10 in decent condition with a good battery.
Voltage drop between igniter black/yellow wire and battery negative is about .3 - .4 volts while idling at 1100 rpm. I also tried connecting a 14 gauge wire between the igniter ground direct and the battery negative, and found no effect, other than eliminating most of the .3 - .4V drop at the igniter ground terminal.
Battery is at about 14.4V at any rpm above about 1200, (45 deg F) measured with both a digital and an analog multi-meter.