On passenger cars, all the manufacturers standardized their electrical systems at 12 volt negative ground in 1957. There were a few 12V + ground systems in 56, notably Ford, Nash, and Packard, not sure about GMC. Commercial trucks like Freightliner kept the positive ground up through the mid-1960's with the idea it made for more reliable grounding and less galvanic corrosion, especially where aluminum and steel were used together.
Jerry
