Given the size of your tank, one option is to use 1X4s and pocket screws. Think about building 4 picture frames (two uprights and two horizontals) and then attaching them at the corners to make a box. This technique only requires a good miter saw, the pocket hole jig, and a drill. It will give you the most amount of space under the tank, you just need to make sure the sump can fit through the front opening.
So here is a quick rendering of what I was thinking as the inner support frame for the tank. Any thoughts?