The holes you drilled in your tank should be the same size as the two holes in the outside overflow box. If the holes you drilled are a little bigger than the bulkhead it's not a big deal as long as the gaskets are making a seal. The seal is not made between the "neck" of the bulkhead and inside of the hole you drilled. The seal is made on the flat surfaces of the tank, gaskets, and the overflow boxes. Hope this helps.