65 Mustang coolant sensor

Just swapped out the intake manifold on my car. Replaced the aftermarket Edelbrock with a '65 Ford cast iron 4 barrel intake. Hooked everything up and it runs great! The problem is the coolant sensor. The manifold had an old sensor in it (early 65 small type) that never gets past the first mark on the temp gauge. So I bought a new one and it immediately goes to hot. Replaced it with a 2nd new one figuring I might have gotten a bad sensor, the 2nd one immediately goes to hot.

Any ideas? All three look exactly the same. The car is not pinging and the radiator is not overflowing so there is no way the temp gauge is reading properly. I put the old one back in and drove it to the Mustang show in Sellersburg, Indiana this past weekend with no problems.

Thanks in advance, MikeR