lets start with "c" first. Due to the fact that you already cut the end off of the tank, I'd move forward with using it as a firebox, but I would not recess it into the cooking chamber as per your original drawing.
If it was still a full tank and I was building a square firebox, I would, but with the two round shapes, its going to be easier and look and function better just mounting them flush. You will also gain more usable cooking area "c" by not recessing it into the chamber. And you wont be trying to but a reverse flow plate to a round tank either.
"A" is the most important, adding that second plate under the top of your firebox will keep the reverse flow plate from getting hot at that end and it will make such a difference in how even your temps are inside the cooking chamber. Also , I think it acts like a insulator, and even though I have not tested this, I believe it uses less wood.
I moved the stack, just becouse I think it would look better like this and will not be in the way if you need to add a counterweight to the door.Also will not take up any internal space.
I would cut a square out of the round end of the firebox and frame it in with angle so that you can attach a flat door. The flat door will allow you to put your intake vents in the door and allow the air to move in a straight line through the firebox into the cook chamber. Even though some of my builds have the vents facing the front, I really dont like to do that. Dont forget to add a second vent on the bottom of the firebox. You will thank me for that down the road on real windy and cold days.