Tank = 4620 ci
FB = 1525 ci +++++
MINIMUM
FB air inlet 6 Sq in .... additional inlet across from the FB/CC opening about 3 sq in....
FB/CC opening = 20 sq in (24)
Area under the RF plate = 20 sq in (24)
Exh stack vol = 80 cu in. = 2.5" ID x 24" above the CC
Some of the above numbers allow for a correction in friction loss....
FB/CC opening
7" radius semi circle 4" high will give you 24 sq in under the RF plate and FB CC opening... which is good...
That also translates to a RF plate 11.5" wide and it should be welded to the top of the FB and stop short of the weld on the other end of the CC to allow for adequate air/smoke flow around the RF plate....
Cut into the CC at the height of the RF plate when you put it inside to check fitment... I would slide the FB into the CC to just past the weld so the RF plate is in the straight section of the CC....
An easy way to get the FB to fit to the CC is... set the RF plate into the CC.... project a line from the RF plate to the conical end of the tank... cut the end on that line.... slide the FB into the newly cut opening.... scribe the bottom of the CC onto the FB and cut out that part circle... while all this is happening, check the RF plate fitment to the top of the FB... If the FB is too high, no problem.. scribe the RF plate to the FB for height and scribe the CC to the FB for the cut out....
Cut out the part circle on the FB and check all the fits.... install drains etc. and go for it...
The above is what I might do to your tank etc..... after listening to all the folks here and tweaking numbers so the smoker works better...
Here is the plan I start with then make adjustments depending on the size of the smoker.... adjustment are made based on friction loss estimates.....
... click on pic to enlarge ...