There are Parts for a '57 - '60 truck here, Group 3, third page:

http://www.vintagepowerwagons.com/pdf/parts/03fuel.pdf

There is a good, detailed "Allpar" article on Sending Units here:

http://www.allpar.com/history/mopar/electrical2.html

As far as the Testing Procedure, there is also something in the Moparts Tech Section, though you have to read through it:

http://www.moparts.org/Tech/Archive/fuel/10.html

There is a YouTube video on "Mopar Fuel Sending Unit Testing":

http://www.youtube.com/watch?v=ZxsNvqyq4Ts

And this "Mopar Muscle Magazine" article on Fuel Sending Units:

http://www.hotrod.com/how-to/engine/mopp-1208-fuel-sending-unit-replacement/