someone could of put the wrong sending unit in just to plug a hole or it was the original sending unit for that motor someone swapped in. test a wire see if it puts 12v out at anytime (key on and off) if no power ground it out the temp gauge will peg out when you find the right wire.
i wrote most of this up a while ago, afaik its good info.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=28592.0
your master cylinder didnt sit right with memory about the manual cylinder. but i have been wrong before 
but to me the brake line coils look like theyre bend back towards the firewall instead of straight up and down. tell you the truth i didnt know any master will bolt to the firewall
Yea I guess the guy I bought it from did some riggin' and left out those details when he sold it to me but thanks man your awesome! I'll get her back right
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