[untamed]

I really like the idea of having the stand stick out a bit into the back room so that you can stand on the edge. I have this on my tank and wish that I had actually made the edge larger (mine is only 6"...8" might be better/easier to stand on). The fact that the stand sticks out and creates a catwalk on the backside makes some of the plumbing more difficult, but it is worth it. I use mine every day. In fact, the bar out front really came to be because I wanted a catwalk on the frontside as well.

Is there no way you can remove the front wall and just build the stand so that it is flush with the wall? Basically, the stand itself would become the wall under the tank. This would create a bit more space under the stand as well.

You mention some sort of concrete lip on the floor...just build the stand right over top of the lip. The vertical beams of the stand at the front would push weight down onto the top of this concrete lip, while the back of the stand would push weight down onto the floor directly.

If the stand is larger than the tank to produce that catwalk on the backside, you don't need any extra beefing up of the sides. You just need to ensure that the stand floor (or roof) is firm. In my case, the top of the stand is 2x6's on edge set about 1 foot apart. Those distribute the load to all sides of the stand. These can easily carry the weight shifted 8" or so from the edges...or just the back edge in this case.

You can see the 2x6 floor on top of the stand in the pics below. It is basically two parts: 2x4 walls...and a 2x6 (on edge) floor that sits (attached by large screws) on top of the 2x4 walls. A structure like this would support the weight even if the stand were considerably larger than the tank itself....just like how your sofa doesn't fall through your upstairs floor just because it is the middle of the room.