I always felt that anything over 6" was adequate to keep from mashing your box side walls. Some hitches have more roll capability than others but you should be fine. I would be more concerned with what made your rear end sag so much you needed an erector set to lift it up. Have you weighed your rear axle with the loaded SB on it? Also a new height measurement would be good now that you are all puffed up.
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.