How far should holes be apart when drilling the back of the tank?

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I am planning on drilling the back of a 180. What is the safe distance between holes. I wanted to drill 2 holes towards the middle which will be 2" apart. The size of the hole will be for a 1.5" bulkhead. I think the hole is approximately 2.5". It will also be approximately 6" from the top and on the middle of the tank. Is this safe? The tank is a standard AGA 180. Thanks