1st stage hitting back of head

[LKunze]

Fred, two things factor into your problem. The first is that you need to position the tank so that it sits low enough to not bang your head on the first stage. What I do personally is set the tank height so that I can reach back and still grab the tank valve. If I can't reach the valve then I know it's too low.
The next factor is how secure of a fit is your BC and does it have a crotch strap? The problem I've seen with most BC's is that they ride up on you while in the water. This is not good. That is where the backplate wing setup is so nice in that once it's adjusted properly it stays right in place and doesn't shift side to side or up and down while in the water even in a heads down position.