Factory drilled corner overflow. That's what most are...for some reason google just showed me a center drilled. 20 long is fine...but when doing a sump you always want to do a "gravity test"...meaning you want to have your tank running with the sump at your desired level, and cut the pump. If your sump is too full already to hold all the water that will drain from the top, it will overflow. With a 20 long on a factory 75 I would say don't fill your sump any more than half, but I tend to conservative. You could probably do a bit more.