Do not attach the large wheels until the wires have been soldered onto the motor terminals, and strain-reliefed with a dab of hot glue.
The battery box is the heaviest part of the chassis; it should be located somewhere in the back end(the square end)
You will need to find a place on the chassis for the Arduino microcontroller, the L298N motor-controller board, the power bus, as well as sensors and other accessories.