Dads50 makes a good point about removing the terminal wire from the sending unit before testing its Ohm value. Like Bill states, it is entirely possible for the aftermarket gauge to require a much higher ohm sender. Chances are, like Mike said, someone probably smoked the coil putting 12v on it. It only smokes for a second and when it's over, you cannot tell from appearance that it's toast. Don't ask me how I know. To eliminate this issue forever, just get one like this:
http://www.classicparts.com/1949-53...2-Volt/productinfo/24-700D/#.VstyyZwrKHsThat one has a white needle, there are red needle ones available to. Point being it's 12 volts natively with no runtz or diode required. Available from all of the truck vendors, so shop around.
