Good evening!
I highly recommend lining it with firebrick. They don't absorb moisture like block do. Just an idea. The more moisture there is in the smoker, the longer it take to come up to temp.
Damper in the flue. I did a steel one on mine. I don't recommend it. Steal and brick added with heat don't mix. At normal operating temp it isn't an issue, but if you ever run your smoker up to 500F plus, it'll crack something ever time, trust me.
Jim, one of our brick builder built a top damper. I don't know all the details, but it looks a work great. If you happens by, he'll be more then glad to give you advice.
Looks like a awesome project. Can't wait to see it in progress!