The shape does look tight but the calculator is based off of volume so it shouldn't be an issue. I would work out where I wanted the racks and then tweak it from there to get the opening as tall as possible within reason. The BP really only has to be as high as the top of the throat but most guys raise them an inch or so and put a shadow plate below the BP for the first 12'' or so of the CC length. That is not a must have but it helps avoid a hot spot at the throat. The top of the FB can be the same height as the BP but the main concern is that the throat needs to be at the highest point of the FB so that the heat can flow out as smooth as possible.
It’s only a 20 inch chamber. I would count on only 2 racks but would want them as deep aspossible.. My bottom rack is about 3-4 inches above the baffle plate.
It should work either way but I would personally go with the taller opening rather than the narrow wider option. Normally I like a wider throat but 2.6'' just seems a little tight to me. Just be sure that there's not a lip on the top of the throat where it exits the FB, it needs to be a smooth transition.