That should work well.
Because of using 2x6 lateral support, you could actually get away without the middle braces, at least the front one. This would allow unobstructed access to the sump. If it was 8 feet long the middle braces would definitely be necessary. A plastic framed glass box aquarium must have solid support on four corners. Yours does that.
The physics of glass then create the supporting structure for all four sides. The top boards must be perfectly flat or just slightly bowed down in the center. The worst thing is to have the boards bow upwards. This creates a high center. That's what causes glass panels to crack.
A full floor for that stand to sit on would be a good thing. This spreads out the weight over a larger area and also allows a very important safety feature, a flood pan. The chance that water will leak out of the plumbing or that the sump will overflow is always a possibility. Placing a plastic sheet or pond liner inside and up the insides of the bottom frame will keep that water contained.
Did I see that the 150 is drilled in two center spots? If so, that's a little inconvenient. Would you like some ideas on how to deal with it? Also, if you are going to paint the back then I would also paint the bottom. To do the job it is supposed to do, the bottom sand needs to be in darkness. I would not paint the sides because there are too many good views from the sides of a tank.
Aloha
Edited by Mark Peterson - June 03 2014 at 9:24am