Coolant temperature Sensor and temp reading

Two parter, First of all I bought a sensor at NAPA and it is way too small for my 88 Saab 900 base. The one that is in the engine is almost twice as long and the threads dont match up on the new one (too small). However the electrical plug fits into it. Anyone have a part number or a link for one that will fit.

Part two- My temp gauge on the instrument panel flips all the way up past high as soon as you turn it on. Common problem? Ground?