Once you find the temp gauge wire you may also have to switch sending units. The sender for the earlier clusters is a different resistance profile than later units, and if you use the replacement listed in most parts books it will make the gauge reading too low.
I'd have to look up the number of the sending uit I used, but when I finally found a matching sending unit (the original parts number is very hard to find) my temp guage now reads about 3/8 scale when the engine is warmed up (195 degree thermostat) and at 1/2 scale I estimate its running about 200 degrees.
Have you checked to see that the black grounding wire is connected to the park brake bracket or other good chassis ground? If that wire is not connected it will cause a variety of instrument panel problems, including flaky turn signal indicators.
Bruce