[dsraven]

here is a site that may prove helpful for you.
55-57 trucks are a little different underneath because the front fenders are a bit longer so the frame is also a bit longer. the rad support is further ahead and also shaped differently. 58-59 are the same under there. they are the same from the front axle centerline back and may possibly use the same front springs as well. the 58-59 also has different hood hinges and mounting because they use the inner fenders as a mount point as well as the firewall. all aftermarket cabs will say they fit from 55-59. just ensure you know what you are dealing with when you order parts thats all. an assembly manual could be worth the money as it should have the frame dimension diagrams as well as a bunch of other pertinent info.