LB,
Welcome to the Stovebolt. Are you trying to use a new electric sending unit with the stock '50 gauge. I believe the stock gauge was not only positive ground, but probably 6 volt.
If all this is true, you are going to have reduce the voltage to the gauge, swap the wires on the back of the gauge and experiment with finding a new sending unit that will make the gauge read properly.
There has been a ongoing thread about this for some time. Those gentlemen will undoubtedly pop in with first hand info.
I have not done this swap so this info might be out to lunch!
http://www.oldgmctrucks.com/photos/GMCWIRE_F350-24.jpgStuart