Very nice! My only two critiques would be that it is SLIGHTLY small for that tank, and if you had any longer or larger pieces that it would be wise to swap them in for some of the smaller pieces, AND that you should definitely give the substrate some slope. Most people completely forget or don't even realize that the substrate is in fact part of the hardscape, and that it should be used properly. I'd try placing the construct you have built there on top of a hill, sloping all the way to the right and left glass panels. It will give the substrate more depth, AND make the contsruct seem bigger without using different pieces (because it would be higher up).
One last thing, Id' pull that rock thats all alone to the right, in towards the structure about 2 or 3 inches, the back a little bit so it's behind it, but NOT out of sight. It's much to far away from the central construct.
Very nice hardscape, and I can't wait to see it planted!! Good Luck!
Edit: oh, and about the HC, it looks a lot nicer on a slope, like I mentioned with the substrate. My scape is actually very similar to what you're planning on going for. This is an old shot, the HC is about twice as thick now, and the grass is looping over the surface like 8"