[bertoni]

I'd be tempted to use a U-tube-like return, to avoid having to drill the tank. Otherwise, remember that when the power goes off, the tank will drain down to the return input level, assuming that's lower than the overflow. You'll need a self-priming pump with that location. I don't know much about return pumps, but I use a submersible, and would likely locate the pump below waterline, if I decided to go external.

Now would be a good time to drill for a closed loop, if you want to use that approach for flow.