Thanks for the advice. I've had bad luck splicing harnesses before, so I'd rather tap a new port and re-use the known good working harness than to hack it up.
I don't have my hands on the head yet, but is it safe to say that I either look for the for the mounting boss on the head, or find the corresponding spot based on my stock '90 head, and then simply carefully drill through until I reach a water jacket and tap to 1/8" NPT?
Essentially. I think a number of #0630 heads had the boss in place and ground - it's just hit-or-miss on whether it's been drilled & tapped.
Bear in mind that standard pipe threads are
tapered, so back the tap out and check your threads at relatively frequent intervals. It's entirely possible to tap too far, and you'll end up either not being able to torque the threads to seal them, or not having the threads mesh at all in the first place!
When installing the sensor, PTFE paste is not indicated - the sensor is self-grounding, and I've noted that PTFE will interfere with that. Use RTV copper, it's got actual copper in and self-grounding sensors (in my experience) still work with this stuff on the threads.
Or, use never-seez (which typically has a metal base,) and tighten to two flats past finger tight. Check for leaks after a day's service or so, and tighten
one additional flat if need be.
Why? If you've heard anywhere near as many stories of this thing breaking when removal is attempted, you wouldn't ask...