I don't know of an easy way to do a close shaping job without touching the stock. But in case you don't already know, a common way to install a new pad to a finished stock is cut the pad close as you can first, like you said, then mount the pad and tape the stock to protect the finish, then sand the pad to final size. Tough part is getting that small fraction of pad left by the thickness of the tape, and making sure the tape doesn't damage the stock and be able to protect it at the same time. Please show us a picture when done if you can and let us know how you end up doing it and good luck with it.