The problem with fasteners is that there were several suppliers to GM at that time and no one bolt is correct. There's Anchor, RBW, TR, SBC, RSC just to name a few, and all the vender bolts that were received were dumped into a bin and grabbed at random. You might have an Anchor bolt, a TR bolt and an SBC bolt holding on the bracket and this would be "correct".
I don't know exactly how many suppliers there were, but I do know there were a lot!
John might be able to tell you what he has on his 69 Z, but again, what he has may not have been what you originally had on your car.
Ed