You may get some movement from the plate at 30" wide while cooking, but your temps are gonna be below 400F so the movement at the baffle plate will be barely visible to the eye. If your concerned about it run a piece of angle lengthwise down the center of the baffle and have it installed with 1" welds on a 2" center. By welding in sections you will minimize the stress added by fully welding it and also creat a nice stiffener.
Again, from my own research there are some discussions and posts regarding the area at the end of the baffle plate. The gap should be equal to the stack cross section plus 10%. Your using a 6" stack so your cross sectional area is 28.26sqin + 10% = 31.06 sqin. So if your baffle is 30" wide, you are only going to want a 1" gap at the end of your baffle plate and tank give or take. Given your tank is round you should be safe to run the baffle to the end of the square of the tank and leave the curved area open. This is what I did on mine and it has been working great..
Only other thing I see is your drawing dimensions aren't adding up so you may want to go back and check them out. If you add up all of the vertical dimensions you only have 42" accounted for. You still have 6" in there to work with.
Looking like a nice unit in the works.