Grant,
Most of the boxcars that the Sumpter Valley had did indeed come from the Utah & Northern. It seems that almost all of the cars were built sometime around 1880, many by the Ohio Falls Car Company. The most common lengths were 27 feet for the earlier cars, and around 30 for later. From the cars that currently exist, it can be ascertained that the Utah & Northern had boxcars in a series from 250 to 400, at least. The Sumpter Valley rebuilt many of the cars between 1914, and 1917. They came out of the shops with the same hardware (except for the 1600 series cars), but most had new numbers. I'll have to find my ICC reprt to get you better details. Well at least this is a start.
Thanks, Taylor