This screenshot from a 1966 Chevy Master Parts Book for Medium/Heavy Duty trucks shows your 1966 C60 came with 11/16" wheels studs/lugs. Granted this is just specifically ID'ing the front wheel studs, but the rears would be the same diameter. Note that they are left and right hand thread per the descriptions.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
