For cars and light trucks you could purchase the 3 speed overdrive option.
I cannot verify but I read that 55 to 59 O/D commonly had 4:11 ratio real axle. 59 and up O/D had 3:70 or 3:90.
I have no idea what ratio rear axle was used in a 1958 step van.
You didn't say which year O/D you have.
Pre-65 4 bolt side cover. 68 up 7 bolt side cover.
Here are some parts to look at. Notice the number of teeth are given.
https://www.classicparts.com/1955-57-Transmission-Speedometer-Gear-Yellow/productinfo/91-958/https://www.classicparts.com/1959-60-Transmission-Speedometer-Gear-Brown/productinfo/91-959/https://www.classicparts.com/1959-62-Transmission-Speedometer-Gear-Blue/productinfo/91-957/The transmission will have an internal steel ring gear that meshes with the plastic/nylon gear as a set.
Once you have the gears that mesh, fingers crossed your speedometer reads the correct speed.
The speedometer drive gears and your rear axle ratio are variables that you have to figure out yourself.
