A lot of builders have an oversized FB and they seem to be able to make the work but I've always stuck really close to the calculators recommendation and I've never had any issues. The FB can be up to 115% maximum, also the stack is measured from the top of the CC so if you mount it down lower on the end of the CC, it would be 20'' plus the length down to the location of the stack. You could get by with adding a few inches in length but I wouldn't double the recommended length. One thing you can do to increase the length is to decrease the diameter of the stack. The intakes can be as large as you want them since they can be closed down. Most guys like to have intakes on each side of the FB so that you can close one down if you get a wind blowing into one side.
Exactly. There are lots of ways to build a pit that will work. Just might take some trial and error. On the other hand, This way will work for SURE. Hundreds and hundreds of pits have been built off these specs.
I always make the stack high enough to keep the smoke from blowing in my face. Mine seem to work just fine. Feldon's calculator says smokestack measurements are not all that critical.
Firebox air intake is not the door, it's your air vents in the firebox. On a round firebox, I have started putting a vent in the door, like this:
I couldn't come up with a good way to put them on the pipe itself, without stuff falling out. You want it down low, so the air goes through the fire. Easier to do on a square box. The one in the door works really nice, too.