[clay68c10]

***I tested this on my 68. It should work for all 67-72.***
I've heard it also works fine on square bodies and newer. I'll probably do it to my 93.

If you don't want to drill your head for the stock gauge sensor there is another choice. A 98 F-body 3-wire sensor will send the proper signal to the PCM and your gauge.
It is a TX111 sensor and PT2319 connector. It threads into the current ECT sensor hole and you just need to add the connector pigtail. Pins A&B tie into the original PCM wiring and pin C goes to the green wire in your truck harness.

The temps may seem hot, but these engines run hot by design. Stock 04 Vette low fan speed comes on at 226F, high at 235, and GM calls 260F hot.

Here's some pics in reference to temperatures. Sorry for the up/down ones.
190F, 212F, 228F (fans just came on), and 260F (simulated with a 55ohm resistor)