Thanks for the info....
After some digging, I did confirm the thread size to be 18mm x 1.5.
Also found the Heli-Coil repair kit for O2 sensors, but the average cost is about $100! That's a bit steep for a one-time repair! Looked for a place that might rent the kit, but had no luck. I did find a 18.5x1.5mm tap at work, and I lucked-out on this one... Looks like the old oxygen sensor took most of the damage. The exhaust manifold threads were a little beat-up at the top, but once I got the tap started, all went well.
FYI, the new AC Delco Sensor has a very fine coating of a graphite and glass bead anti-seize. I read on another web site that standard anti-seize compounds should not be used because the sensor needs a path to ground thru the exhaust manifold, and a "liquid" anti-seize may interfere with the path and cause false readings???