It should work since the Sound card is only single slot.
But the metal backplate on my graphics card takes up a lot of space and could obstruct the soundcard.
According to specs, the Soundblaster Z is 5.4" x 5" x 0.9". I'm going to assume based off the pictures that it is 5" from the PCIe mounting bracket to the end of the soundcard.
All you need to do is measure from the PCIe bracket of your GPU to the blue GPU Reactor on the back of the card. I would have measured this for you but I sold my Lightnings a few months back.
My gut feeling says that the Soundblaster Z will not fit unless you pop the GPU Reactor off (which you can do easily and will not affect the cards performance whatsoever).