Awesome!!!!! A UDS on steroids!!!! And a thick skin.
If I can get caught up by the end of the holiday I will play with a few numbers and see what happens.
You may be able to use the fitting on the bottom for a ball valve, and have a multi-purpose air intake/drain.
Right off the top of my head I am thinking its on the small side but it may do the trick. If you used an elevated ash pan.
Even if it needed more air. I think I would do the intakes into the bottom within the edges of the skirt. Then maybe have the ball valve below the curved portion of the tank.
Not that it will work or cook any better, but it would be sleek and stream line.....unlike a drum with stuff sticking out totally unhidden.
Another if it was me thing.....
I would flange the lid before you get it cut all the way off. If you don't weld you can always use a light gauge steel and rivets to attach the flange.
I also would not use a fire door.
These units will run for an extended period of time one a single load of fuel.
Alot of guys use the threaded bung fitting on top of the drums for there smoke stack. So I think 3" would be plenty for yours.
Good Luck and pls keep us posted.