1.5" for sure.
Let me elaborate...
You should use the flow calculator that RC has to see how the head and plumbing will restrict flow and what the actual flow through your display tank will be. I think that with 12" of overflow, the 1" bulkhead will be the bottle neck. I don't think a 1" bulkhead will keep up with 400+ GPH gravity fed.
BTW I like your avatar. Is that a picture of your tank?
