411 S3 temperature gauge sender

[Green411]

Has anyone got a part number/supplier for a 1/8 NPT temp gauge sender that gives the right reading on a 411 S3 please?
I had to move mine when I fitted Holley Sniper, as the Coolant Temp Sensor for the ECU has had to go where the original temp gauge sender lived. So the sender now has to go into the 1/8 NPT horizontal port just in front and to the left of the original port.
I read that the colour of the insulator denotes the resistance but that doesn't seem to be the case. The original was black, and a Jensen Healy uses a black 1/8 NPT sender so I bought one, and it read c. 60-70% of the original.....
Thanks
Mark