You mention the sump was used as a qt tank at one point so what chemicals, if any, did you medicate with? The sump looks to be an all-in-one tank. I take it there are slots or holes allowing water to fill the back/sectioned off area? Is your skimmer an hob
? Will the pump fit comfortably in the largest sectioned-off area in the sump?
If so you can have the return fill the back chamber that could lead to your skimmer and use the entire front as a refugium. Otherwise, since your sump is on the right (looking from front) locate the overflow box on that side of the tank, the overflow feed water to a sectioned area where your skimmer would reside (right side), use baffles to provide a water mark for your skimmer and as a bubble trap, the last baffle should be large enough to accommodate a 4" sand layer, and then baffles at the other end where your return pump is.