You DO want to read the hottest place in the head... when the vehicle is stock that hottest place is in the back of the head on the line that runs to the heater core. When you do a reroute you want every single sensor back there again because that becomes the main water route.

 

I honestly don't get it.
I've got two sensors: one in the back of the head (where now sits the thermostat) and one in the front, in the mixing manifold. The only condition that the readings are different is the warmup, and the highest reading is in the front of the head

 

You are confused, the highest reading can never come from the front of the head. That is directly above where cool water from the radiator comes in, how can it be the hottest part?

The sensor in the back of the head is (maybe?) the dash temperature gauge, if you were me I would get a GM 1/8 NPT coolant sensor, put it by the thermostat, then move the old little dash temperature gauge to the front, or just throw it away...

I don't know how you have a sensor in the mixing manifold, but the front water outlet in the head is not the mixing manifold?

 

The bottom line is that the current placement will "work" only in the sense that the car will probably not blow up from having it there.
But if you want the most accurate reading, you'll need to put it in the back near the outlet.

That's basically it.