You can definatly remove it. I have done so with an oscillating multi-tool. Use a few layers or tape to protect the acrylic sides, back and bottom of the tank. You want to cut as close as possible, buy not damage the sides and back. The closer you get, the more you risk marring the sides, but the less work you will have to make it look nice after. Once removed you need to sand it down and smooth. A bit of creativity may help here to speed thing up. A dremmel or router with a support that let's you straddle the acrylic to be removed, but keeps the cutter a fraction of an inch away from the side back and bottom panels. This would make the process quicker and cleaner. After that sanding to remove the final traces. You will want one of the acrylic polishing kits for this.
How nice you want it to look will greatly affect the time involved.
Another option would be to build a rock structure to hide to remaining bits of the overflow box.
