Oxygen sensor and headers
 

I got an interesting answer from a really nice porsche mechanic(no not an oxymoron) in regard my question about locating the O2 sensor for a 3.2 conversion when the Cat has been replaced by headers.

The sensor needs to be downstream from the exhaust flame plume, or it will get burned up, so the perfect answer is paint the header and see how far along the paint gets burned up.(the racer answer)

Realistically place it down where the headers join. Although this measures only three cylinders, ok as long as all cylinders are working generally well, placing it further away and therefor cooler will only richen things a bit erring on the safe side.
Certainly better than running without the sensor in terms of optimal engine control.

I would not worry about the temp as we have an O2 sensor on our 1987 930 and the exhaust gets red hot when doing time trials and the sensor has never faltered. I installed one on my 914 just at the collector and it seems to work okay too. Good luck.

Quite frankly, I would not worry about the O2 sensor. I have a DME 3.2 in my 914 and I am not using the sensor. This way it works on open loop control. By adding the sensor, you go to close loop control, thereby leaning out the engine. Sure 14.7 to 1 Air/Fuel ratio is the optimum for fuel economy, but not for performance. For power, about 13 to 1 is the best for HP. Unfortunately, that is in the non-linear range of most O2 sensors.

If you have to have O2 sensor, I would use a Bosch O2 sensor. When I worked @ the factory, we tested these things @ aprox 900 degrees Celcius (aprox 2000 degrees F).

Another option is to install 2 sensors, one on each header. Then, send the average to the DME. Also, develope a comparitor ckt so that if the difference btwn the 2 sensors is greater than a certain value, an alarm comes on. Just the controls engr in me thinking

Anyway, good luck with the project.