I'm pretty sure the driveshaft is the same between manual and auto, unless you're switching turbo and non-turbo, or 2+2 and 2-seater. The turbo driveshaft is bigger around, and it might have different splines. The 2+2, I think, is longer. (I'd have to check my service manual to make sure of this, and it's not where I am.) Definitely open the tranny up, though, and check for any unpleasant surprises. I recommend draining the oil before you pull the tranny, if it's in the car still, since otherwise you'll have a bunch of oil coming out the hole in the rear.