OK I see you are leaving the 2" exhaust open, my bad. I would think you need at least a 3". Look for a custom muffler shop, they should be able to cut an sell you a 3 ft chunk of 3" pretty cheap. It will be galvanized or nickel plated but its OK for the exhaust.
One 3" will give you the same 3.7 square inch exhaust area you have with the four 3/4" inlets.
Weber 26.75 OTG w/CI grates & SJ/WSM Mini
Home built 24"x72" reverse flow stick burner trailer with 18"x44" grill w/CI grates and upright SS gas oven/smoker.
Home built 48" fire pit with a 30"x30" Santa Maria style ranch grill.
Home built lump charcoal retort, iGrill & a bunch of other cooking toys.