Honestly, I would never, ever, ever run a tank with a hang on overflow box. They're way more trouble than they're worth. Overtime, bubbles accumulate in the U tube, and unless you're good about purging them, you'll get reduced flow and all sorts of problems. They're incredibly noisy, they're unreliable, and they're not particularly efficient. You'd be far better off drilling the tank.
The big fear with a hang on overflow isn't usually that the power will fail and the sump will flood -- we know how to deal with that. Syphon breaks in the return, plenty of head room in the sump. The problem is that the U tube can fail, and you then pump the sump into the tank with no syphon down, and it overflows the tank. Been there, done that.
There's a couple of ways to go around this. You can put the pump on a float switch which will turn it off if the level in the tank gets too high, or if the level in the overflow box gets too low (depending on which type of float switch you get). You should throw out the single giant U tube that they give you, and replace it with 2-3 U tubes, and religiously purge them of air. A redundant one ensures that if something clogs it, you'll have less of a problem. (You can also add a second overflow box). Beyond that, its just a regular, religious check of the U tube for any obstruction, and removal of air bubbles. And living with the fact that it may eventually fail.