Do you have money for another motor? The 3.4 has claimed more engines than any other pulley. You are running a stock tune, a restrictive exhaust, no valvetrain mods, and a smaller pulley? And you ask how do I know you have KR? Not trying to flame you- just stating the obvious.
Another thing, how did you chop off your resonator? did you weld a straight piece of pipe in its place? If you did, then you could do the same for the U-bend. And buy a bung from Mcmaster (about $2.00) and weld it in the pipe for the O2 sensor.