Realistically, if the tank isn't drilled now, I'd probably not worry about it. You can get an external overflow for not too much money and they work fine if taken care of. I haven't used one, but friends have used them for years without issue. Yes, there is a risk that the tank could break, and lots of the corner tanks used tempered glass anyway, which you can't drill. Keep the drilling in mind for the next tank (yes, there's always a next tank).
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Brad
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