Kevin,
According to a search of
GMPartsWiki, your rag joint coupler clamp bolt with the slightly odd looking head (see pics below) uses the same sized head in both truck and car applications through the mid '60's.
Only difference was the length of the bolt depending on the end vehicle application.
I checked the bolt head on my wife's '66 Chevelle this morning (1st pic) and confirmed a 12 point 7/16" standard socket slips right over and grips the bolt head just fine. On a double check, I found that a 12 point 11mm socket also fit, but was "very snug", which may lessen the chance of damaging the bolt head during removal/reinstallation. NOTE: 6 point sockets do not work because of the bolt head design.
Second pic is a brand new replacement bolt pic grabbed from a web vendor to show a clean view of the bolt in question.
Hope this helps???
Dan
