If you can make the front cross beam doubled up 2x6's, then you could do this:
put one (or two) 4x4 post 1/3 of the way from one side. This one is permanent and non-removable. Once you get the sump under the stand, add a second (or paired) 4x4 post 1/3 of the way from the opposite side. this way you have a beafy beam to brace the tank horizontally, and you will have 2 (or 2 double) posts in front, one removable. that should be strong enough, I *think*.
What is the tank made of? Is it a plastic stock tank, acrylic, glass, plywood? That will also have a bearing on how the legs of the stand need to be positioned.