ok, so i replaced my fuel sending unit on my 85 and at full i m reading a half a tank. So i am thinking that I might have installed it in correctly.
mines is sitting for left to right so the plug comes in from the right sitting horizontially.
. ill try to put of a pic of what i have .

 

This is common when the electrical connections on the sending unit get corroded. Try wire brush or sand paper to clean up the male and female ends of the sending unit connection really good.