1963 Dart Bad Temp Gauge Sending Unit?

Your instrument voltage regulator is inside the fuel gauge. The connection from limiter to fuel gauge is inside that gauge. The connection from limiter to temp gauge is via a fuel gauge post and the printed circuit board path to a temp gauge post.
Here is the most likely location of intermittent contact you describe.
One contact post of your fuel gauge has a odd metal clip in a larger square hole through the circuit board instead of a speedy nut like all the others. This was for venting some heat out of the fuel gauge and away from the circuit board. It was deemed unnecessary and replaced with a nut for 67 and later. They actually placed a bakelite cap over the nut and stud which bottled the heat without issue.