One option that has not been mentioned in this thread is the Butt Transplant. If done nicely, most of the glue joint will end up in the bottom of a row of re-cut checkering, and thus should be all but invisible.

http://www.picturetrail.com/sfx/album/view/16839920

This procedure is also covered pretty thoroughly in Shotgun Technicana. I'm thinking about trying this out on a VH Parker that I bought cheaply because of its' bad buttstock. It has about a 12" length of pull, however a Butt Transplant would only be for experimental purposes as it has also been pinned through the stock cheeks. Doing the Transplant would also give me a pattern stock with pull and drop dimensions of my liking.

If the head of your stock is sound and has good inletting, this might be something to consider versus adding an extension and doing grain painting which might eventually show being repaired.