Good sturdy stand, slight overkill maybe but it's each to their own with stands. The top of my stand is similar to yours, though in 2x4's. All I did at the point you're at now is throw a piece of standard three quarter inch ply on top, no glue, and screw it. Then I put a sheet of styrofoam on top ready for the tank, that was 6 years ago and I haven't had any problems. Make sure you get a level on the top, not just front to back and side to side,but diagonally too.
If you aren't using styrofoam you need to countersink the screws/nails. You don't want the bottom of your tank anywhere near any screw/nail heads.
The only time you'll experience problems with warping is if you continually get the wood wet, maybe through clumsiness on your part during water changes, otherwise it should be fine if kept dry.