this link may also have some frame dimensions that reference face of cab (firewall) to rad support mounts, cab dimensions, etc. both sites would be good to bookmark or download the info you need later. some of these have pages missing but the dimensions you need would be roughly the same for 55-57 and 58-59. the last couple of years the frames were longer out front and the rad support mounts were a little further forward so be aware of what year you go by if that dimension factors into what you are looking for.
http://chevy.oldcarmanualproject.com...esto/56134.htm