Tim, the attached link is to the truck specs for a '57. Scroll down to page 135. It shows the 16" optional rims for the 3100's and 3200's with the same lug pattern as the 15". Rim size is 16x5. Would be the same for a '58.

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1957-Chevrolet-Truck.pdf

Matt