6-ply truck tires were the commonly used 16" light truck tires in the late 60s when I lived in VA (as specified by GM). The 15" light truck tires from GM were rated 6 or 8 ply.
http://chevy.oldcarmanualproject.com/chevyresto/51150.htmI suppose if you wanted white walls back then, you needed to buy 4-ply car tires.
If the rubber is brittle, checked, or cracked, forget about them. Even if the tires look OK, be careful.