Quick minor correction to my post - 8 kilobytes of code not 2.
One further thought - this question arose as a part of discussing adding an O2 sensor to a set of headers that did not have a factory installed bung. Depending on where one is able to retrofit a bung, there is merit to discussing whether or not the chosen location will be suitable to heat a non-heated sensor sufficiently to get accurate readings. That is a topic I have no opinion on. The ECM will drop to Closed Loop at the same time based on coolant temperature regardless of the O2 sensor; the O2 sensor does need to be hot enough at that point to send accurate readings to the ECM or the air/fuel mix will be out of whack.
I'm running the old emissions legal Edelbrock 6879 headers on my '87 SS with a standard O2 sensor installed in the factory provided bung. It bounces around rich/lean as one would expect so long as my carb is well tuned.