If the head size is 11/16, the thread diameter is 7/16. It the head size is 3/4, thread diameter is 1/2. You would know better than anyone about the length, since you have the brackets and the car, while we do not. I would imagine 1" length would work, but again I don't know what you're working with. Mine is a 64 and I believe the bumpers bolt on differently.
