On modern EFI BMW's (E30, E36, and E46 that I know of, I don't get more modern than that) the sensor that kicks on the fan is in the radiator while the sensor that reports to the gauge is in the head. Food for thought...

Cool thanks much. I have an I.E rad, so I think I can take out the plug and put in a sender, the sniper has 2 built-in fan relays and temperature manager that I can use. 

I am actually taking a third option.  The 1975 has a fitting in the manifold that I am having re-tapped as a 3/8 npt. 

You can get the E21 or E30 318i coolant divider with extra bosses in it, but I think your solution of just retapping the manifold should work fine also! (Don't spend this much, but as an example: https://www.turnermotorsport.com/p-35743-11531255158-genuine-bmw-part/)

So here is what my fabricator was able to do.  I have a FLIR so once I get it up and running I can see how well the sensor is placed.   It just seems like it will be tidier

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