[Lightning]

You can use one of those locations on the header to install the leading O2 sensors.

If you don't put the trailing O2 sensors behind the remaining catalytic converter, you WILL get a Check Engine Light (because the ECU checks to see a difference in O2 readings between the leading and trailing sensors) and it won't go away.

As far as re-flashing the ECU, it depends upon your country's emissions testing laws...