Overflows can be tricky to DIY properly and are such an important piece of equipment I wouldn't mess with constructing one. You can buy an Eshopps HOB overflow that is very compact and extremely reliable for not much $$ and have no worries about floods.
For the return pump, do not return flow to the sump. This is harder on the pump, uses more power, is louder and creates more heat than simply placing a ball valve in the discharge line of the pump and throttling the flow down in that way. Submersible centrifugal pumps only draw the power required for the work at hand so adding head, cutting flow in the process, means the pump is working less and is more efficient. When you return unused water to the sump you have already pumped it so the punmp works harder, draws the power required to pump that water and creates the heat for those watts consumed then you just dump it back where it started from.
I can't stress enough how much I would like to see you use a different return pump than the Quiet One. I have literally had two Quiet Ones go up in flames, sparks, electrifying the tank, smoke and all. They are horrible pumps, run extremely hot, draw more power than better comparable pumps, and worts of all have lots of documented trouble retstarting after a power outage or feeding time. I would not wish one on my worst enemy and certainly not a fellow reef hobbyist. My display dropped 2 degrees after replacing the 4000 with an Ocean Runner 3500 and the power on the Kill A Watt meter dropped 15 watts from 68 measured watts to 53 measured watts with no other changes. Plus I never had to disassemble the pump anymore to spin the impeller with my finger until it started again. Absolute junk in my experience. One was a fluke but two sealed it for me. Mag pumps are a little more reliable but are very loud, draw higher than normal power and run hot, they do restart though.
Look into better pumps like Eheim, Ocean Runner , Octopus, WaterBlaster, Sicce and Tunze. All are more efficient, handled head well, draw less power, are quieter and create less heat. None are as inexpensive but they will last and last.
You really do not need a return pump that large anyway, the recommended return flow is 3 to 5x your display volume after headloss and the 4000 is a 900+ GPH pump at 0' head and still quite a lot of flow at the normal 4-5 feet of head. Your sump will be whistling like a jet with that velocity travelling through it and you will have massive microbubbles even with baffles since you will have little detention time for them to dissipate.
Even 5x your display volume is around 250 GPH which is where I would suggest you be, 250-300 maximum and make up the rest with powerheads which are much more efficient.
A 1" overflow is more than sufficient, I use a 1" on my 100G with a Water Blaster HY-5000 pump wide open and have no problems. The return plumbing can be 3/4" or even 1/2" at 300 GPH with no issues. Keep the fittings, bends and valves to a minimum to reduce headlosses and on the overflow keep it as straight as possible with no sharp bends, I actually preter reinforced vinyl tubing for my overflows with gradual sweeps for direction changes and minimal barbed nipples for attaching. I usually oversize the vinyl tubing one size, as in 1.25" instead of 1" to account for the smaller inside diameter of the barbed nipples and fittings to reduce restrictions.
