The first component we'd suggest looking into would be your Buick Regal's EGR system. The EGR valve is the number one component used for NOx reduction. Second would be fuel control (oxygen sensor and MAP). If you have the engine running lean your going to come across high NOx issues. The larger throttle body is definitely not helping as well. Technically your Buick Regal's manifold pressure sensor should pickup on the difference of vacuum due to the greater amount of air entering the engine, however your Buick's ECU calibration may not allow the extended range.
We'd recommend ensuring the EGR system works, the EGR passages are free and clear of any carbon build-up. Second make sure your Buick Regal is in proper fuel control. This means the oxygen sensor is sending the correct signals to the ECU and the ECU is commanding the proper amount of fuel to the combustion chambers. And finally (if possible) replace the throttle body with the stock one.
