IMO a rounded groove like you have in your block would seal better than a square groove * , but i would still follow the standards, what i mean is that if I were you, I would have made the groove a little shallower so that more force would be applied on the o ring it self.
Edit * it think like this because all off the o-ring would surely be in contact with the walls, in a square groove, one cannot be sure if the oring is in contact with even the corners of the groove
anyone is sure about this one??