Here's a photo I took of my GR1-SF of inside the body, when I was investigating how big the block behind the tailpiece might be. The block in the foreground (on the left) will be the support for the bridge posts, so the screws for the Bigby need to be wide, but not long, as they're only going through the top, into nothing.
With video, it's easy to upload to YouTube, then post the link here.