So that picture is the inside of the tank shot from the front?
Everyone is confused because 1) nobody has seen a setup like that before and 2) you haven't explained the problem very well or taken any decent pictures that make any sense.
Your quickest option would be to just cap those holes and seal them off entirely, and run an HOB overflow. Or you can make a closed loop out of them. If you really have to use them as an overflow, then build yourself an overflow tower around them like a reef ready, cut out the box that's there, and modify a durso to fit in its place. The other line I would use as return, and make it's exit come over the 'new' overflow tower box you'd have built around it.
It's either that, or get a new tank. Considering the original problem, if the overflow is emptying faster than it can 'take in' water, it's most likely due to the fact you have 850gph of pump trying to suck that much water into a pipe (with air) limited to 300.
Honestly though, more pics would help. It looks to me like the exits are on the lower *back* half of the tank. Like it's a reef ready tank, minus the exits being on the bottom of the tank, and minus the overflow tower built around with the proper pipes coming up to the surface. More than that, it looks like they were closed loop lines that were modified.