Here are some links to a map from 1881. As it states in the novel, the factory was at the corner where Marshall Street meets the Hoosac River, directly across from where Mass MoCA is now.

Here is the whole map.

And here is a higher res blow up of the map. In the lower left corner, near the edge of the map and alongside the river is a large squarish brick building with a smoke stack. That's the shoe factory. A bit down the street to the right you will see "Marshall Street." The blow-up doesn't include MASS MoCA, but it's on the original map.

I just found this - a better link.  If you zoom in you can see both Arnold Print Works and Sampson's factory. Here is a screenshot with the buildings marked.