Howdy. Any luck yet?

I prefer analog meters. No crappy "compensation" or auto-ranging to interfere with my raw data and continuously moving needle.

If your voltage is not swinging on a new sensor, you've got a problem with the mixture.
When I test my O2 sensors, it's merely for "does the voltage swing? and how quickly?". I test with the car running. The voltage should swing from just above 0V to just below 1V. You could call it a quick and dirty test, but it's testing the essentials and not fussing over the unimportant details.
Positive lead on the O2 sensor output, Negative(black) lead grounded somewhere I've scratched a fresh ground path. Rev the engine a couple times.
I usually goose the throttle a couple times, then hold the engine at 1500 and let go of the throttle. Each time the engine speed changes, you should see the sensor swing one way for a brief time, then return to a steady cycle somewhere between 0 and 1V.

A new O2 sensor is not likely to be bad off the shelf, even if it's a mustang sensor (Can you explain that more). Replace your plugs with NGK Coppers (or something else if you prefer), Pull the vacuum line from your
FPR and check for liquid gasoline. Try testing the O2 sensor again.

By the way, are you checking the O2 sensor output with the O2 sensor connecter to the harness, or with the sensor wire disconnected? Do you get different results if you try with the wire plugged and unplugged?
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1990 740 Turbo, on its way to stock specs, maybe beyond