AndyJ,
Did you ever move forward with the B5 on the Thinline? I recently got a MIM '69 Thinline (2006 model year) and was thinking of doing the same thing. I shined a flashlight into the f-hole and noticed that the center block on my Thinline does not extend all the way to the end of the guitar. It looks like it goes just far enough to accomodate the strings as they pass through the body and I'm wondering whether the body will be able to handle the stress of the Bigsby.
As an aside, the necks on these guitars are fantastic.
