The rear crossover pipe/steam vent reduces local hot spots in the rear cylinders by allowing air and steam to escape. The LS1 came with the crossover pipe, but was removed for the LS6 as it came in conflict with the larger intake manifold. The ports are still in the LS6 engine block but they are blocked off. 
. When I had the manifold off to change the valve springs, I decided to add the crossover pipe. I simply used the pipe from a truck, GM part 12605716. I only had to slightly bend the pipe to refit the LS6 manifold. I put on a T-junction and routed the hose back to the coolant expansion tank.