KevinS - The data in the 1955-59 FAM is a conglomeration of info put together that doesn't have all the individual year details correctly listed. The info klhansen provided comes from this
1958 Chevy Truck Data PDF which is specific to the '58 model year and more accurate than the FAM for sure! A further check of 1955.2-57 frame data in the GM Heritage site documents shows those where all listed as 178.05" overall length frames. So it appears there was a minir length change with the '58 model year.
If you are looking for a bit more detailed info in another source, pickup a copy of a 1958 Chevrolet Truck Shop Manual like this
original printing one I found doing a quick web search. There are also a number of reproduction books being offered from lots of vendors, but many are wanting lots more money than this one. Good luck on finding the correct frame!
