Those years listed above are approximate - GMCs did not sell by year, they sold by series.
GMC did not seem to make year-references (at least in vehicle/chassis serial numbers) during the years below.
They sold vehicles of one series until they sold out and then started selling the next series. It is quite possible that your "Y" series truck was sold in 1956 as a new truck.
Yes, 6v electrical systems were used in Y-series GMCs but 8-cylinder engines were mostly 12v in GMC Y-series trucks.
Tim
