John,
I have a 29 gal tank on a mediocre, store bought stand. As I sit here looking at it, it occurs to me that it probably *would not* sag, due to the fact that the glass itself is a structural component. It occurs to me that so long as the tank is primarily supported on its ends, you in fact *might* be able to remove the center panel on your design.
A big however, though. How you going to keep the unit from wracking without a back?
The torsion box concept is for stiffness only.
I'd completely rethink this whole thing, and frankly, I'd be thinking square steel tubing.
Your design is very modern. I'd think about how to encase a steel framework with panels.