Topless in Toronto

I have changed the box and shifter in my C5, seemed ok for awhile,couldn t get it into reverse,no big deal, take it apart again and I see the output shaft on the box is not secured by the clamp,ok,tighten it a bit more and it is really tight now,work it back and forth a few times,and it starts spinning inside the brass clamp,everything is aligned properly,changed boxes again,same thing,what am I missing here ? If I tighten the bolt in the brass clamp anymore I am sure I will strip the threads. I can t see how the shaft can spin inside the clamp when the bolt is in the halfmoon slot, but it does. Any ideas would be appreciated,I have done the search and did everything according to the video,to no avail. The ZO6 shifter and box that I installed has supposedly 3000 miles on it, the output shaft on both boxes show no wear.
Thanks Al