The worst case scenario is what is done all the time. You can get a copper fitting that goes from male threads to 1/2" copper. Sweat the threaded piece onto a separate piece of 1/2" copper. Then put the piece into the spout, snug it up, and mark it where the end of the tub spout is. Take the piece back off and cut if off at the mark. Now place it against the finish wall next to your existing copper stub. Mark both pieces somewhere about in the middle, cut both and sweat together using a sleeveless coupling. Thread the spout on after wrapping with teflon tape. If the spout isn't quite snug enough when it lines up where it is supposed to be, wrap a few more turns of teflon tape on and try again.