Here is a website the has a lot of manuals to view.
OLD ONLINE CHEVY MANUALS

They don't have a shop manual for 1940 trucks but they do for the 1938 and 1941 trucks, which are very close to your 1940 if not the same.

1940 CHEVROLET TRUCK OWNER'S MANUAL

1938 Chevrolet Shop Manual for All Passenger Cars and Trucks

1941 Chevrolet Shop Manual for Passenger Cars and Trucks

On the same website there is a lot of info in the Chevrolet Master Parts and Accessories Catalogs also.

1916 - 1929 Chevrolet Parts for Four Cylinder Models (Effective July 1, 1940)

1929 - 1932 Chevrolet Master Parts Price List for Six Cylinder Models

1929 - 1941 Chevrolet Master Parts Price List (Six-Cylinder Models)

1929 - 1954 Master Parts and Accessories Catalog

1929 - 1957 Chevrolet Master Parts & Accessories Catalog

1939 Chevrolet Accessories - Modern to the Minute Brochure

1951 Chevrolet Accessories Manual

1952 Chevrolet Ready Reference Time Saver

1956 Chevrolet Accessories Price List

1958 Chevrolet Parts and Accessories Catalog

You will want to check out Brad Allen's Original GM Salesmen & Mechanics Training Film Strips they are a treasure trove of info.
Note: Under the Album tab there are two pages, scroll to bottom of page, to view page 2.
Most of the film strips were produced using black & white film.

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC