That is one big overflow box and it will put a lot of stress on the back panel, pulling it backward. Need to be sure you have enough bracing to support it. Else you will end up with a cracked back panel.
I think its better to remove that overflow box, and make a new, much smaller version.
And do not use those copper nuts where it will be exposed to seawater. Copper is bad for certain shrimps and hermits.
That stand needs a lot of extra trusses. Triangular support in the corners to prevent it from swaying. Total water volume for display is about 540L. add that to the weight of the glass, rocks, sump, equipment and it is not something you would push around on wheels. Please be sure that the wheels are at least capable to carry 300Kg each. More is better. I think everything will be close to 1000Kg, and I will not push that around. Rather drain the system at least 50% before doing that.
In the pictures I cannot make out how exactly will the water enter the overflow box. Is the back panel cut out lower in the middle?