383 WITH AFTERMARKET TEMP GAUGE

When I got my car it had an adapter between the sender and the intake that caused the gauge to read 15 degrees lower than the engine really was.
The adapter was only about an inch long, but that's how much off the gauge was because if it.

Now, the current question.
There are so many different gauge and manifold manufacturers that it is almost impossible to tell someone what they need, but because of the above you need to try and use a gauge that does not require stacking adapters.
The one I have on my motor now does have an adapter for going from 3/8 in the manifold to 1/2 for the sensor, but the sensor is about 2 inches long so it is enough that it physically reaches into the coolant flow and reads correctly anyway.