No most people will connect a lenth of PVC
on the inside of the tank that runs up to around the top of the water level. Then you can use lock line
to adjust the flow where you want it in the tank.
For the overflow inside the box most people use a durso stand pipe
. It helps adjust your flow and keeps the tank from making so much rushing water noise.
Using the holes to connect to your sump will be much better then an overflow box. They can be a pain to adjust and can cause some problems. If the holes are already drilled i would use them.