I used the Not shared version, and I replaced just the copy in the SF root folder.
AGuyWithAComputer
I'm almost done. What you did, sort of, worked out right, but it was not complete. I'll fill you in on how StreamFab seems to do it... Take the whole contents of the Bin subdirectory of the unzipped ffmpeg-master-latest-win64-gpl-shared.zip file and place that in C:\Program Files\DVDFab\StreamFab replacing what is there. Then copy just the FFmpeg.exe file from the Bin subdirectory of the unzipped ffmpeg-master-latest-win64-gpl.zip file and place that in C:\Program Files\DVDFab\StreamFab replacing what is there. I have no idea why they mixed the shared and static that way, but that's what I've found they do, and I have done the same.
Now, here is where I had to make a decision. In previous versions (6.1.6.7 being the last version I worked with prior to 6.1.8.3), they took the whole contents of the Bin subdirectory of the unzipped ffmpeg-master-latest-win64-gpl-shared.zip file and place that in C:\Program Files\DVDFab\StreamFab\YouTubeDL. In 6.1.8.3, they don't do that, for reasons unknown. I decided to keep adding it at this point and will explore it some to see why they made the change. Most likely, in future versions, I will do what they do, as that has been my goal, all along, to follow their examples and methods but with more updated program versions.