The depth is fine, as I mentioned in a previous post, the depth isn't nearly as important as the width. I've read your post several times and I think I understand what your planning on doing. If I'm correct your planning on pouring a 6 foot wide pit...period. Then instead of pouring walls for the H pit design your going to have the H pit made out of sheet metal with metal legs supporting the center section. Is that correct?
If it's OK with you, I'll send you a PM and perhaps we can discuss this further. Just for anyone else that may be reading this thread, I should mention that a water drain at the bottom of the pit would also be recommended.