Nick
The Chevrolet Master Parts Catalog, 1929-1946, indicates the braces were the same for 1939 and 1940 1/2 ton trucks.

The photos I have for ‘37 and ‘46 1/2 ton trucks might help provide an idea of what you’ll need. These braces are incorrect for your truck. I will email you the photos to avoid creating fog and confusion otherwise.

Hopefully, others will post photos of the correct ‘39/‘40 braces in reply to your request.