My 69 pickup had a similar problem where the needle was at 1/3 full scale at 185 degrees or so. I replaced the wire wound calibration resistor on the back of the gauge with one of another value from Radio Shack. The needle then pointed straight up at the same temp. I don't know for sure, but 73-87 trucks may also have a replaceable calibration resistor.
Lots of good info here:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.0
An excerpt from the link:
COOLANT TEMPERATURE GAUGE PARAMETERS
For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω
For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω
For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω