Originally Posted by buttburnersbbq
Scrape the offset and build a reverse flow. Once you have a RF you will not cook with anything else. There are pit calculators on line you can find to size your firebox, stack and opening fro firebox to pit.
^^^^^Agree. You will be very happy with RF performance and ease of cooking. If you go RF, by definition the firebox opening is well below the cooking grate and you count on the RF baffle to contribute radiant (very balanced) heat. I'm just guessing, but at 20" diameter and doing RF you will probably wind up with a single cooking grate (i.e., RF baffle about 6" above the bottom, cooking grate at center line or 6" above the RF baffle and about 6"-7" top clearance (maximize this!)
; add in the thickness of the baffle and cooking grate structure and you are pretty close to 20" ID). I would be tempted to extend the exhaust stack into the cook chamber about 1/2 way from the top to the coooking grate. Just my $0.02. Good luck and welcome to the forum.