I would but I can't figure out how to from My phone but I can tell u I have a 500gph return pump with about 4 feet of head room which would be 425gph I don't have a valve in line with it. On my overflow I have a ball valve on each of the drains. The overflow is a aqueon model 220
pics will help, and you may need to just add a hose line to introduce air to break the full siphone, witch has to be tuned to work silently...
but pics will help me to try and give you a solution.
They won't upload to mu CPU for some reason but I'll try to explain the best I can,
From my overflow I have two drains a ball valve on each that goes to 3/4 tubing on each to a Tee that goes to the muffler on the sump the return is about 425gph
If so, you should not drill a hole at the top of the U-tube! If you do you will lose your siphon entirely. (Some of the advice you are being given is for a standard overflow in a tank that is drilled).
and as mentioned upthread, slowing down your return pump will also help. If you don't have a ball or gate valve in your plumbing on the return line, you might want to consider adding one.