If you can find a spot to screw straight into the sump this would be the best place if not put the sensor in the
oil line going to the filter, this will be straight from the pump.
Oil goes from pick up through the pump to the filter then the cooler if the thermostst is open. The
oil then goes to the engine to do it's job and then drain back into the sump.
Going through the cooler first without a thermostat and then into the filter may cause the filter to "blow off" on cold mornings.
Easy no drilling, weldiong required and you will get an accurate oil temperature reading.