position of O2 sensor - 4AGZE

[cosworthpower]

Hi I have a set of 4-2-1 extractors and need to weld in the 02 sensor bung. Its a standard type 4 wire sensor.

Question is where is the best place to place the sensor?

I have a factory ECU if that helps

[TERRA Operative]

After the final merge, before the cat.