I'm not sure, but post #10 on this thread says they are .660
http://67-72chevytrucks.com/vboard/s...d.php?t=375734
I was going to go the scientific route - Take one of my hooks to the LHS (not a big box store) and find Grade 8 bolts that fit and are the correct length to include the thickness of the frame, washers and nut... Then drill the correct size holes for those bolts.