Does this mail box have enough depth underneth to attach wood for excisting mount or will holes need to be drilled on the sides?

Hello, yes, you may use a 2 inch (in depth) wooden board to mount the Centennial Mailbox to an existing mount. The pre-drilled mounting holes on the Centennial Mailbox measure 4x10 inches. Don't hesitate to contact us directly at support@architecturalmailboxes.com if you have additional questions or need additional dimensions. Thanks for reaching out!