Chevy dwell settings changed from close to 40 degrees to the low 30's when the electrical systems switched from 6 to 12 volts. I have an idea that GMC would have also followed this pattern. It has to do with the current flow in the coil primary and the ability to develop enough magnetic saturation of the coil core to produce a hot spark- - - -more voltage, less coil saturation time needed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!