Are you really sure the exhaust is airtight in front and around the O2-sensor? Have you tried installing the sensor in the other tube for comparison?
I've installed 2 AFR-gauges in my '60-NewYorker with the rollercammed longram intake engine in it. But the more I try to tune that system, the more I just want to go EFI with that setup. I've got some clips of the gauges in the '60 on my Youtube channel to view.