With mine, I ended up getting a male-male fitting that screwed into the sender hole, and then just cut it off with my lathe then drilled/tapped it for the 1/8" fitting. Machine tools are fun :-)
If the coolant is too low, you will still get some kind of reading from the sender, but it will not be accurate at all. It will most likely stay low for quite a while, but if your coolant is that low you will also be able to feel the difference with your hand by grabbing the upper radiator hose. It will feel comparatively cool, and when the engine is cold it will be really easy to squeeze.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.