I assume you want to access the 24" controller portion from the front?
If I am understanding your questions correctly, it would probably be fine to use a design like I have pictured below and as shown in the attached eDrawings model (viewing software is a free download).
The boards on the inside of the legs are 2x4's to keep them from protruding past the edges of the 2x6's. All other boards are 2x6's as you stated. Cross-braces under the top surface could be either 2x4's or 2x6's, and are probably mostly for additional stability than actual support. Cross-braces are also added to give support to the middle span of the plywood, but this is not really necessary for tanks with rim frames, since all the load is on the bottom glass and transferred to the outside edges.
...that is, assuming your tank is a traditional glass tank with top and bottom framing trim. If this is the case, almost all load will be transferred through the stand legs on the 4 corners, so the uprights in the middle are just for extra security for material failure and might be able to be left out entirely (though I recommend all that are possible without obstructing access).
I do not have a lot of experience in tank stand building to give you confidence in my suggestions, but I work as an agricultural structural engineer and have some carpentry experience. I just recently built my own 90-gallon aquarium stand to allow for a 75-gallon sump, so the back of my stand has one upright in the middle and no uprights in the front to allow for placing and accessing the sump from the front. My setup is less than half the weight, though, and only 48" long, so it is not directly comparable. The main difference in my design versus your design is that I ran 2x6's between the frame top and the top plywood along the full front and back length as well as one in the middle (as shown in the model). These were screwed into the front and back boards of the top frame to make them into a sort of angle to increase stiffness and support strength without uprights. I would recommend the same for your design to ensure the boards do not bow, but this is probably over-designed and might be just fine if you use just the plywood and fasten it to all boards touching the underside of the sheet. I plan to custom-build a countertop from 3/4" plywood and laminate sheet to put over the 2x6's on top of my stand frame.
Using plywood directly on the floor would be fine structurally, assuming the floor is strong enough to support a full sump directly on it (I have little doubt this is your case, but I am no professional). The main downsides of plywood on the floor directly are that water may seep under the plywood and have no way to dry while ruining the flooring underneath, and there is no good way to level the stand (though you can do this during construction or on the top surface).
...(Edit) and I forgot to mention that I would include at least the diagonal braces on the ends to prevent frame twist. The braces should go from top corner against the wall (if possible) down to bottom opposite corner. Diagonals in at least one opening on both front and back would also help stability, but are not necessary if you use 2 or more screws vertically in each 2x6. The screws spaced apart should offer enough moment-stiffness to prevent the frame from twisting and collapsing.
Feel free to ask questions about any aspect of this!
This picture is looking from the back with the 48" opening shown.
Here is another angle showing the back from the opposite corner and showing the front braces better.
