Yep, that's a body shim. I've replaced the front fender on my Cougar and forgot to put this back in. Not a big deal, but I would check for squeaks and gaps if you're trying to figure out where it went.
i don't remember taking off and body parts though?
ill check to see if it was something started related.
It is possible that they used that as a shim for the starter as well. It would be used to bring the bendix gear in the starter more inline with the flywheel. I've never seen them used in that fassion by the factory, but it is possible that a mechanic did it after the fact. Just a thought.
iv had the motor, trans, or transfer case out of mine a good many times and never remember seeing those used in the driveline at all. im with everyone else that its a body shim but why it would have been near the trans is beyond me.