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As for alternative temp sensor locations, I have an auxiliary coolant temp gauge mounted, and I plumbed into the back of the coolant cross over pipe. I, too, wanted to maintain my factory coolant temp sensor. On the pipe, catty-corner on the other side of the factory sensor, you will find additional metal real estate from the casting. This proved to be a great spot to drill and tap for 1/8 PT sending unit.
If you decided to go another route, any one of the ports you plug a block heater into would suffice, so long as you had the right conversion plug to fill it, so the new plug could take a temp sensor.
Here is an example of what my set up looks like for my water temp gauge:
Coolant temp gauge circled in yellow above.
The red circle is where I tapped. And the red arrow shows the route I took with the wires.