Just a learning point:
When a HG goes or there is an "air pocket" in the cooling system, the temp sending units can actually see low numbers or spike super high. They rely on fluid contact to give proper readings. If there is a pocket of air around them, they may actually drop and show low temps. The other side is sometimes the air pocket will get super heated and cause steam in the air pocket and may cause it to spike, but it will go high, then low in a matter of seconds. (This is more common on other makes than the 1FZ)
The reason your overflow tank level wasn't changing is because there was no vacuum in the radiator to suck or draw fluid to/from the overflow tank because your system is no longer sealed. The system could draw air in from somewhere to equalize pressure.
It could be leaking into a cylinder, have a leaking hose, or a leaking oil cooler, although each will show up with different symptoms. That's why it's so important to PROPERLY diagnose the root cause, rather than just say "yeah, it's your HG" and fix that because maybe it's really your PHH or the bypass pipe O-rings that failed and leaked out your coolant, then the overheat condition caused the HG.
Since it was running that low for an unknown period of time, you may want to seriously consider changing your water pump when doing this, because the coolant is a lubricant for the WP bushings on most vehicles.