My vote would be a totally different direction. Hire someone to punch holes in the tank. Overflow boxes are so risky :/ I know a few professional installers, and they all poopoo overflow boxes every chance they get. If you are set on overflows, I'd go the route that would allow you to modify as little as possible. Really, you shouldn't be deriving a ton of flow from your sump loop anyway, and should be using alternate flow sources to get the bulk of you water moving. My 2 cents