You will have a little siphoning action to the height of the weir in a power outage. I happen to love holes in the top of my tank, but even with holes in the top of the tank, there could be siphoning until the water level is below the return nozzle. We added four holes to the top of our 300 gallon tank.
There really is no RIGHT way to plumb your tank. Your return can be on the top or the side of the tank. I have an irrational aversion to drilling the side of my tank for fear of a bulkhead fail. My big brutes have managed to break three bulkheads on the top of the tank by ramming into uptakes. Because I did not specify anything about my overflow boxes, this is what I got on my big tank:
The return lines come up through the overflow boxes and enter the tank between the weirs. I feel safe with this because if a bulkhead should break, any leakage will be contained by the overflow box. And besides, there is no room for return holes on the top of my tank. I had the tank builder put eleven holes on the top of my tank for the mechanical filtration I have planned.
Drill your return holes where you feel the most comfortable. I like your external overflow box.