I've got a 1.25" gate valve on order. I'm going to install it in the horizontal 1.25" pipe (below the one you have x'ed) that goes around back of the stand.
If I can get it to work OK, I plan on adding an additional bulkhead to the side of the overflow box, up high, and running a separate line all the way to the sump. This would be my safetly overflow line.