USAaristo

I have a 2000 Lexus gs300 and it's been running rich since I did the swap. I hopped under the car and realized my o2 sensor is just hanging next to the exhaust and there's no place in the aftermarket exhaust to put the sensor for the JDM aristo swap. Where should I put it so the car stops running rich? Pre or post cat? And does anybody know the bit size I'll need to drill and put the sensor in? Thanks in advance.