Thats what Im talking about, good info.
I take it then the firebox is square. Are you limited to the 8.5 x 7 you have stated in your design? Would you be able to go wider and make the height shorter? Reason I ask this is the more room between the meat grate and the opening the better, as its best to have some room from where your tuning plates start to the grate. Unless of course that won't be an issue, then never mind.
On your stack design, I would go with the 4" pipe, and don't see a need to run it any higher than 24" out the top. Some people will argue that running the stack down to the grate is the way to go, others say the opposite, to have it flush with the top. I am of the later style. I run a reverse flow and its not as common on them as it is on a cross flow. The reason I went this route is that in order for the smoke to escape the main chamber, it has but only one exit, the chimney. Which means that the whole chamber from top to the bottom of the stack will be filled with smoke. And sometimes that smoke can become overpowering and stale and thats not what one wants coming in contact with his meat. The smoke should just gently roll by kiss the meat. But there is a way to have the best of both worlds on this one, and its a modification that many on here do. Run that stack only an inch or 2 in from the top, and if you have to, use dryer vent to do the rest. Adjustable and it won't take up grate space like a solid pipe would. Also, if you ever put in a second rack, the chimney pipe is not in your way. You simply use a large hose clamp and clamp the flexible dryer vent to the stack. Another reason that the stack is run down to grate level is to keep more heat in. In a cross flow design, once the heat enters the main chamber, is starts to go up right away and the opposite end of the chamber can be quite cooler then the end where the firebox is at. Having the stack all the way down allows for more heat to stay in, but since you are going to go with tuning plate, you can adjust to a degree, how much and where you want to focus the heat. It is more or less a perforated reverse flow design.
Your tuning plates should start at the top of the cutout of your firebox. Look at the designs I included here. The first one is from a site I had bookmarked about a year ago. I am also including the text that came with it.To help understand tuning plates, the gaps let heat and smoke rinse as shown below. The wider a gap is, the more heat and smoke rises in that immediate area, the hotter that area becomes. Since the firebox end is much hotter, the gaps at the firebox end are usually very small to reduce the heat at this end and help guide more heat to the far end (in fact the first tuning plate in the Liberator is welded to the firebox and the next adjustable plate is butted up against the first with no gap). Although the first gap has little or no space between plates, sequential gaps are slightly bigger than the one proceeding.
Typically tuning plates are used to even out the heat across the pit but they can also be used to create "zones". If you were to want one area hotter for cooking chicken and a cooler area for slow smoking brisket at the same time, these zones can be made by opening the gaps or removing a tuning plate while closing the gaps at the opposite end.
This image was made my me. Nope, I didn't make a career out of graphic design, go figure
What I wanted to show in this one was the firebox in relation to the main chamber. If you can get your hole dimensions shllow and wider, it would be best as the heat and smoke are entering from a lower point, if that makes sense. The area with the X's in it is the area that would be cut out.