Bass Ackward Fuel Gauge

[Turtleman]

This is driving me nuts.. It's at the point that I know it's right in front of my face but I don't see it.
In my 66 C10 if the ignition is OFF the Gauge reads full,, turn the ignition ON and it goes to Empty... Change the wires,, right?... Guess again.
A few time's during the daily drive I've noticed the gauge bump to a readable level... Sending unit replacement?... yeah,, OK maybe. But why the reverse reading on the gauge?
I just recently acquired this and am noticing the previous owner's work as I get more and more into it so I want it to be "right"... some day soon I'll tear into it, but right now I just really don't like running out of fuel when I need to get some place.. LOL.
Anyone else run into this?