GMC still used 6 volt positive ground in some second series. I own one. It is a 150 with a 248 and was born 6 volt. Parts books and maintenance manuals don't mention it. But the one I have is a second series. The 6 volt battery box is on the firewall, but only large enough to accommodate a group 1, not a group 24, and everything about the truck is 6 volt.
