Well, you could put the gutter around 3 sides but not the front and unless you tilted the tank slightly to the rear to ensure that the tank overflowed into your gutter and not your living room It would still be a problem. Tilting the tank, however, is never a good idea and creates its own set of problems, not the least of which is catastrophic tank failure.
Your options, as I see it, are:
- have your tank drilled
- get a better overflow like a Lifereef. Even the best external overflow can fail however.
- install a safety cut off switch: A float switch and circuit that cuts off your main pump when the tank gets too high or your sump gets too low.