I don't know about the F-Body transmissions, but I went through the same thing you did. Picked up a T5 I was told was from an F-Body (WC, had the roller countergear bearing & everything). I looked up the tag (about 3/4"x5") #, turns out it was out of a '91 S10 with a 4-cylinder. I don't know where the trans. mount will be in relation to the stock one (an F-Body T5 will fit with minor work to the stock crossmember pad), but to bolt it up to the engine you'll need an adapter bellhousing, which will likely be $350 or more.