As part of my electric fan install and grill cover project (see Performance forum) I also wanted to replace the temperature sender unit with the correct one. The one they put in when the new engine was installed was a cross-reference replacement, but they are not correct. It always read low, when comparing actual temperature read from the radiator inlet neck with an infrared thermometer. It also wasn't the correct connector.
The correct GM part for 1973-1978 C/K (all styles, all engines) with gauges is GM #1513321. These were used on all GM cars from 1957-1967, and some GM vehicles for another 10+ years after that. They have not been made in years. Even the GM cross-reference replacement does not have the same resistance curve. You can sometimes buy an NOS #1513321 on ebay if you have a couple hundred dollars you have no better use for.
This of course drives people like the Corvette restorers nuts. So one of the restoration parts houses built an exact replacement unit with the correct resistance curve. They also have the correct connector. I installed them this morning, and on my test drive running errands to town, the gauge at correct operating temperature (per the infrared thermometer) reads exactly two ticks of four -- straight up.
Here are the parts in case you have a need. Parts were shipped in a timely way, parts received were as shown, and installation was uneventful.
http://www.americanautowire.com/shop/temperature-sending-unit-5028http://www.americanautowire.com/shop/connector-housing-temperature-sender-con