Looks good so far. I just finished pretty much the same build you're working on. Here is a pic of mine.
Without input from others, I plan to do the same. Take the length of one 3" chimney and cut it in half for having two, maybe add a couple of inches.Thanks storeman. I'll be short so I'm not hijacking the thread. My stacks are approx. 2 5/8" inside diameter and they are around 30" tall. My cook chamber is 60 gallons like the other one in this thread. That's why I posted a pic of mine so maybe he could get ideas. This was my second build. My first build was 120 gallons and I believe I used 4" pipe for that one. You should be okay with two 3" pipes. Just use Feldon's calculator to see how long yours should be. Just remember that two 3" pipes isn't the same as one 6" pipe. I figured how long my pipe needed be and divided it by two since I went with double stacks. Not saying that is the proper way but that's what I did.
My progress has slowed way down on the grill project, I thought by the time I was to the firebox damper a solution would have been developed. But no...I sat outside for about an hour just looking at the firebox and cooking chamber and how to connect the external linkage to open and close the damper for the firebox, take a look at the photos attached, any suggestions you have would be greatly appreciated.
I may be showing my ignorance, but why would you want a damper for the firebox? I get the impression that you are considering an internal damper. You have the adjustable dampers/vents on the door or on an adjacent wall plate and you have a chimney which can be dampered if desired. What are you trying to accomplish?
Jerry