What Configuration For 40gal Breeder Overflow and Return?

[gregzz4]

Either will work.
I'm for a center overflow with Y loc-line returns on either side of the OF box and pointed to both the corners and the front. This will help eliminate dead spots in the back corners. Then you can hide your powerheads down low beside the box instead of them hanging around in the corners.
Drawback to centered is ease of access.
It may come down to your personal preference for maintenance and looks. If you will be viewing the tank from other than straight on, you'll most likely not like corner boxes.
Also, decide on what kind of flow you'll want before designing your box(es).
There's a drain/overflow calculator for you here that will help you decide how long the overflow edge needs to be
And you'd need to calculate your return pump for my suggested idea