According to the calculator the stack is spot on.....but the fire box is bigger than it should be......I'd run it as is and learn it. I had a pit similar to that one and fire management was key, with smaller fires/coal beds working best.
So that would make your firebox too big and your stack too small. As mentioned, run it a few times to see how it operates but now you know a few areas that might need adjusted.
Decreasing the firebox volume is pretty easy with a few firebrick or a false bottom and the stack is pretty easy to extend it needed. Honestly, I would be more concerned with the size of the throat as that can mess things up in a hurry if too big.