DSLR camera image transfer using USB over IP
Solution 1:
USB over IP won't help you in this case: The problem is, that Windows can't handle the 40 cameras via USB, no matter if it is direct USB or USBoIP.
I had a similar project once (Video surveillance), and what worked fine is to use a few Rasperry Pi (or similar) computers with ca. 4-8 cameras each, that download the image via USB and pass it on via Ethernet. This has been working flawlessly for years now.