Need help with overflow plumbing

[Ryan_Lap]

If I completely turn the ball valve in the closed position imitating a clog the stockman can't handle the flow and will over flow.

As for the drawing. The red thing is the ball valve and the circle going into the sump is a bulkhead. The over flow is an external overflow. I ideally would want to use the Herbie method, but with the 2 lines joining into one this doesn't work. Not sure what I can do from here.