Minimum Clearances when drilling an overflow

[sphelps]

You can try using the smaller drain for the main and the larger one for the back up. Make both as large as possible by using thinner bulkheads if possible (although I'm not a fan of the thin ones)

Also an alternative for the back up is to use the smaller drain but use a larger diameter pipe than the bulkhead (if 1/2" bulkhead, use 1" riser pipe) in the overflow to slightly increase the amount of water flow it can handle. You can also use larger pipe below the back up bulkhead as well. This will eliminate most of the pipe friction and increase flow.

Lastly just throttle back your pump to match what your drain can handle.

I would use one or more of the above methods over re-drilling the tank.