That's way bigger that would be useful. Your bulkhead size is what dictates flow, and that flow rate will determine minimum overflow requirements. If it were me, I'd make it only as deep (front to back) and tall as required to fit bulkhead into and go full width. My glass holes model is about 8"w x 3.25"d x 5.25"h with 1.5" bulkhead and moves around 800 gpm in my 90g. If I bump return flow up much above that, my level stays a little higher and surface skimming doesn't work as well (oils and stuff in surface).