The new sensor/sending unit is installed, problem solved! She heats right up to the bottom of the green zone and stays there nice and happily.
I also checked the freeze point of the coolant. Basically I had the whole system flushed out with distilled water, and drained what I could out of it. Per the chart on the peak antifreeze bottle I added 6 quarts of straight coolant and filled it the rest of the way with distilled water, which is supposed to achieve a freeze point of -34 in a 14 quart cooling system. After driving it around to get the coolant mixed evenly, I took a sample with my digital antifreeze refractometer and came up with -33 so I came pretty darn close to my target!