If you are doing a coast to coast on the end, it won't really weaken the glass; especially if you are cutting it as a slot or eurobracing. I assume that on a tank this size you are are having eurobracing or steel brace around the top? It may even be possible to do it on the long edge as well if you design it right.
If you design the external overflow well, it'll just disappear. When I look in my tank through the water (rather than above at the rimless edge) the only give away that I have an external overflow is the way the light refracts through the water going over the edge. If there is no backlighting I don't see the overflow at all.
I spent countless hours over-engineering my overflow (like, who calculates the thickness of the water layer spilling over the edge, really?... me.
), so if you want to have a chat about how you could do it I could give you all the tips I learned along the way.