Help getting started

hi, 

I've plugged your figures into the calculators posted by DaveOmak, and the below is what I came up with.
Your FB/CC opening will need to be at least 103 sq inches.  If your using the two sections of pipe, use the segment calculator to work out the height of your segments.

http://www.1728.org/circsect.htm

From the above, needing at least 103 sq inches, the below are the figures I got.  The segment area is half of what you need, because your total area will be made up of two identical segments, from the cc and the fb. 

All of the calculator and material above is courtesy of DaveOmak. Look up the post "
[h1]Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use.. rev5.. 6/19/15......[/h1]
Note the figures are minimums.  Your Firebox will already be larger than the min.  You should consider increasing the other sizes above the minimums, but you probably want more experienced builders jumping in here to help.

1/4" is way thick enough for capping the ends and reverse flow plate,  I use 3/16" most of the time to save $. (diffference building for yourself or building to feed the family.)

I use 2  x    #16  (1.6mm)  4" tubes for my 24" cookers.  Again the little bit of heat dissapation between #16 and 1/4" thickness is not going to change to performance of your cooker much.