My instructions for my wideband A/F kit says you can put the sensor anywhere in the exhaust as long as it is a heated sensor and no less that 30 inches from the end of the pipe so it can't get fresh air to it.
If it's not a heated sensor, then put it right after where the collector and as much before the engine case vent tube as you can. Like right after where the headers bolt to the rest of the pipes.