I would try setting up your tank on a platform supported by 3 piers of cinder blocks, oe at each end and one in the middle. Place a sheet of plywood at least 5/8 inches thick on top of the block that matches the outside dimension of your tank. The three stacks should be perfectly equal in height so that the plywood lays perfectly flat with no effort on your part to level it. At that point, get help and get that tank on top of the plywood and well matched to its edges. After all is in place, go ahead and fill the tank. A typical tank is supported mainly by its bottom "trim" and if you give it a flat surface to sit on it will do just fine. Whoever built the stand for my 120 gallon tank had no idea about the strength of materials so it is far too heavy for what it needs to do. It is built of 2x4 lumber held tightly together and sitting side by side on solid wood piers. The darned thing weighs a ton and is not truly flat without me adding a shim under the pier at one end. Your design is superior, if a bit crude.