WHO can shorten a C-Body axle housing?

Agree, a machine shop that does blocks - heads and build engines would not be the place to shorten a differential housing.
You want a race car fabrication shop, a place that does welding.
Go to a local circle or drag track and talk with some crews/drivers and get recommendations.
Shortening an axel housing is not a difficult job for a person with experience and the right equipment.