Yes this looks good to me, I too could add the steel corner braces, my tank is 6'X6X4' high, I am going to put steel straps across the top at the center both ways to help in any spreading at that point. I have bolted most of the points that I think could use it, before I add the insulation I will probably and more lag bolts to the floor on the edges. I have already got my EPDM liner, it is 15X15 and I also got a 10X10 for the top. I purchase the liner here http://www.webbsonline.com/catalog/liner/index.html I thought they were reasonalbe and I got my liners fast, I did not have to buy a whole roll of EPDM roofing.
Steve
Steve