Fuel Gauge

[GMC_Aaron]

I recently bought a 1972 GMC Super custom truck and the fuel gauge has a problem. It seems to work slightly, but when I fill it up, it only reads 1/3 tank. I pulled the sending unit and saw that it needed to be replaced. I thought that maybe that would do the trick, it didn't. I checked the electrical and everything checked out ok. I did notice that when I pulled the wire connected to the sending unit, the needle went past full. I'm confused now. is there a way to calibrate these gauges? Or is there more to it? Can the needle be taken off and adjusted? I haven't attempted to take the gauge out yet, but I have checked everything else to this point and I have only the gauge left. It seems like it has its' range of motion, but I was thinking that maybe the needle was messed with at some point in time before I got the truck. I don't want to tear the dash apart until I get some more insight. Thanks.