Can't see all the 90* bends, but TQ is right. I would wack off the exhaust so it does not extend any lower than the top cooking grate. The 3.5 inch exhaust is too small but keep the altitude (35 inches is pretty good for draft). Also, the intakes (2@2x6=24 sq in) are right at the minimum limit, given your cook chamber size you need intake and exhaust to be more like 30 sq in.
Bottom line (IMHO) for a 120 gallon tank, a 6 inch exhaust = 28 sq in is enough but you need a little more intake (2@ 2x6 = 24 sq in) try to add another 2x6. Either cut the stack flush with the top of the tank or not more than the top of the upper cooking grate and keep that stack at 36 inches+.
Best of luck!
Got a bunch of cooking toys and a custom metal fabrication shop where I spend my time building all sorts of smokers & outdoor cooking gear.