I suggest using a pump or pumps that can pump slightly more gph than your overflows can handle... then slow them down just a touch with ball valves...
Keep in mind head pressure, plumbing diamter, length of plumbing, bends in plumbing, etc will cause restirction and slow down the actual flow rate... I suggest your pump/pumps can slightly exceed the actual flow rate...
Be as creative as you want to be or can be with the placement of the filter returns to give yourself maximum filtration. It's not going to be easy to vac the bottom of that deep tank.