Originally Posted by Mustang_man298

This one's an easy fix. Lance is right about their functions. I ran into this same thing when I put the 617 in my '80, my fist thought had been to take out one of those coolant plugs and replace it with the bushing and sensor from my old engine, only the plug was frozen in, yours will probably be too. I did try to glow it without the sensor, the GP's did work from what I remember, but they didn't work correctly.

Here's the fix:
First, did either of the car's have an EGR setup that used the plastic v-shaped vacuum switch on the t-stat housing? Use that housing, remove the switch, put a plug in the smaller hole if you have 2 holes. The sensor will now fit right into the housing, will not interfere with the t-stat, and since it's above the t-stat it will still read the temp in the head. Then you can either solder an extension to the existing wire for the sensor, or do what I did. I didn't want to cut up the wiring, so I went to the wrecker and found a newer model (I think 82 up has these) look at the GP relay, there will be a single round connector with a single wire on it. I don't know what it was for (it only apeared on the later models) you want the male half of that connector, and you also want one of the sensor connectors from the wrecker too. I then carefully popped the connectors open, and soldered in an extension wire, which now plugs to the factory wire and the sensor and looks clean and factory-ish. I even took some of the plastic sleeving fron a GP wire too to put on my new wire for the look (& protection). I then ran it along the upper A/C hose in front of the engine with a pair of zipties. My only dead giveaway that it was added was that I only had white zipties on hand, factory would've been black,lol. (you could even snag a couple of the factory reuseable ones while you're at the wrecker.