so your cook chamber is 36" diam by 144" long?
That is not a small cooker! use the Feldon Pit Calc and whatever the firebox to CC opening is make the gap smaller in square inches than that opening but no smaller than the square inches of your stack size. EXAMPLE= 8" diameter stack = 50.24" sq
The bigger the Gap the more evenly it will cook at higher temps. The smaller the gap the more evenly it will cook at lower temps.
Put the BP above the top of the FB to CC opening.
The bottom grate will be at least 2" to 4" above the BP