If having a overflow chamber, doesn't really matter where you drill as display level is the height of the weir.
As for the pump, as long as the your overflow can flow the max output at head of the pump, water will find it's own level.
Get an idea of how much you want the flow through your sump then use this
calculator to determine hole size.
RC have a
head-loss calculator and there's a
pump guide on RDO.