I'd second Darryl's thoughts about doing a butt-transplant versus complete restocking. And by the time you pay the costs for adding a wood extension, possibly bending the stock, and adding another $800.00 or so for fake grain painting to cover the glue joint that will often reappear as the wood shrinks and swells... then you are well on your way to the price of a complete restocking job. And if you go that far, you will be very upside-down on a Miroku boxlock.
Another nice thing about the butt-transplant option is that you can often find a very nice and high quality blank for far less money simply because it may be a bit too short for doing a complete restock job. If you can beg, borrow, or buy a copy of Shotgun Technicana, there is a very good description of the process.
