Hi Casey,
A miss match of an inch is quite a bit. It indicates to me perhaps the parts you are working with may not be right either because they were damaged or they are the wrong parts. But, I would like a little more information.
Are you replacing the fenders because of collision damage? Are you replacing just the outer fenders or are you replacing both the inner and outer? The inner fenders or as the parts catalog calls them "fender skirts" are also different between 1955-1957 and 1958. Do you have both inner and outer fenders for 1958? Information about parts interchange can be found at Old Online Chevy Manuals. Just look up parts numbers for the various years to know what will fit. This is one very valuable resource.
http://chevy.oldcarmanualproject.com/Alignment of front end sheet metal is one of the hardest projects on these trucks so I give you a lot of credit and wish you a lot of patience.
