have you thought about a rack? you should be able to get all that on a 48inch rack.
I personally would go with 4x4's instead of the 2x4's, would add much more strength to the setup.
I thought about it briefly but my main concern would be one of my ornery boys trying to scale it and pulling the whole thing over on himself. That would be a great option if I can anchor it to the wall though - certainly solves the "wood getting wet" problem.
A lfs near me uses blocks, but puts the 2x4s and 2x6s through the holes in the blocks. In the warehouse they have tanks 120 gallons and bigger three rows high using this method and it works great.