Where do I mount the sensor?

I bought this
Upgrade Black 38mm Water Temperature Sensor 1.5" I.D. Radiator Hose 1/8 Npt Temp | eBay
and drilled and retapped to 3/8" NPT. Or you can but this
Black CNC Car Inline Radiator Hose Connector 3/8 NPT Temperature Sensor Adapter | eBay
which is more money. After seeing Post #5 I may have made the wrong choice.