Gavlar, if you have a !/4 in. its enough. You just need room for the fire brick to expand and contract with the heat. You'll be fine.
You can always have your doors custom built. Lucky for me, my brother built mine. He has a fab shop and I didn't have to pay. If you have them custom made, make sure the frame is at least 3in. angle iron. That way, you can use the top of the frame as a lintel for the brick to go over the top opening.
Looking forward to seeing you build. Its a shame everyone I'd love to meet, live so far away.... If you have questions, just shoot me a PM or post here. Been a busy summer. I'll answer as soon as I can.