Try to get rid of all those spacer plates between the side plates by incorporating them into the inner side plate by bending the flanges in from the inner side plate. Might also think of building a low profile drive base and bolting on a frame with the robotic mechanism built on the frame. Doing so you can reuse the drive base design and build out a frame with the other parts for the robot.
Consider a bent sheet metal tray instead of the cross braces to connect the two sides together. As shown all the forces will be at the corners of your robot. It will be hard to square up and maintain a rigid frame with the cross braces.
Take a look a some pictures of our 8 wheel drive sheet metal chassis.
http://www.spartanrobotics.org/galle...Season&bac k=
Roy