The '40 book is not the best of the resto series, but it does provide many pictures of car details that I think will help a lot with your jigsaw puzzle. Nothing that is specific to commercials will be shown, though. Commercial and truck people are way behind in available resources!
The '38-9 book is absolutely great, but of course a lot of stuff changed between '39 and '40. Still, though I have never owned a '38 or 9, that book is so broadly informative I refer to it frequently.
If you can run down 1940 or so chassis and body parts books, you will find then useful too...your owner's manual can supply a surprising amount of info, too...the pictures of underhood might give a view of battery setup, for instance.