I'm sorry this post is late, I built a smoker out of one of those beasties myself.. I had no issue with it "springing or sagging". I made the lid's back cut first. I then welded my hinges onto the lid and body before making the rest of the cut. Alignment was perfect. I put my fire box below the main tank, 1/4" steel 4 ft long 16"square with an access door on the end. My actual "firebox" is a pick-up truck fuel tank with one long side removed. By building it separate from the firebox proper I can totally remove the fire at any time by opening the end door and pulling the fire out with with a hook. This makes it VERY easy to add more wood or totally remove the heat if necessary. The sliding door also acts as the damper. Inside, I have two movable heat deflectors that run the full length of the smoker. They are controlled by levers on the outside of the tank. I can roast over an open fire or close them off and smoke. It's an ugly thang, but extremely efficient. I have cooked at least 50 pigs an that beast. It is very easy to controll.
500 gal. Carnivorous Spirit tank smoker, 22.5" weber, UDS, 1 gas weeny warmer