What rear end is this?
 

How can I tell what rear end this is? It's a ten bolt out of a 58.
https://cimg9.ibsrv.net/gimg/www.for...42416cdea.jpeg

Ford 9 inch. There is a tag on it on the 3rd bolt to the right, starting at the top center bolt. Post all the numbers from the tag. The main number will be the first, partially obscured by the nut on the stud, and it will be "3 50" or some other number which will tell you the ratio.

Now given the age, it could have been rebuilt a dozen times and have any ratio.

The tag says 3.73

Not saying it isn't but most of them I've ran across in the 3.7 ratio were a 3.70. 3.73s were a ratio many Dana 60s had in the 3/4 ton

x2, 3.73 is not a factory 9er ratio. See the chart about 2/3 of the way down the page:

Ford Axle Ring and Pinion Assembly How to Guide

Can you post a pic of the tag? There is a date code on it along with other info that may tell the tale.

Scroll down to FORD DESIGNED REAR AXLES ~ Example of F100 9" rear axle tag from 1957/63 Ford Truck Parts Catalog.