I got a little nervous after the amount of effort it required to get the aquarium onto the stand, so I ended up purchasing steel angle iron and using it to fasten the front Cedar rail to the cubes. For good measure, I also reinforced the front cube edges and the back rail. I calculated that each of the cubes, with their four vertical side panels make up approximately 6 square feet of surface area in contact with the floor. So 24"x3/4"=18sqin or 1.5 sqft, multiply x 4 sides and get 6 sqft. Dividing the total 12sqft by 2500lbs of water gets me about 208lbs/sqft which seems reasonable. I checked the stairwell and found the floors in the building to be approximately 12" of concrete which is good since we'll have a total of 500+ gallons of water in the apartment by the time we're done - the benefits of a loft in a converted Sears warehouse building.
I drew up some diagrams of the stand so far:
Note: I'm not a trained Engineer, so use your best judgement when building your own stands.