Also I recommend a back mounted pony cylinder as it is out of the way. Yes I sling too, but have been smacked in the face during a giant stride, have had the bottle hit my mask while doing underwater flips, and have to reach around it for the SPG.
I am trying to imagine how this happens...Do you have it clipped off on both ends? I can giant stride and hover upside down, do flips, etc with a slung 80 and it never moves, it would be impossible to hit me in the face. That is with a standard harness, not rigged for SM.