I’m thinking of doing a closed loop or dual overflows and running two return pumps that would utilize the holes the closed loop was likely using. I might still run a closed loop though. It depends on the diameter of the two holes and the research I need to do regarding the closed loop.Adding, you can add a sump... but.. working the closed loop is something you'll have to look onto if its what you'd like to do.
Adding a closed loop isn't difficult. Not sure the best ideal pump is.
Figuring out your overflow box is another step.
This tanks going to be an interesting project if you get it.