With a CPR overflow box do not exceed the flow rating of the box with a bigger pump, it will literally overflow. As for the gate valves they go in the line going to the sump, just above the sump. This will help you control the level in the box so you don't get that sucking noise. Also if you go with a CS 102 that is rated for 1200 gph and a pump that is only rated for 900 gph you will need a gate valve to slow down the flow form the box to the sump in order for the pump to keep up.
Hope that makes sense.
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