definitely go with the OB inside the tank. much safer and easier to accomplish. Additionally, if you went with an outside box, you would still "need" on inside for proper surface skimming. With the box inside the tank, you create a weir, as large as the length of the box, which will pull the water off the surface and into the overflow.
You should do a bit more digging around to find more ideas and concepts, to fill out your plan. Check out melevs, calfo overflow, C2C, etc. HtH