The stock is hollow all the way back to the pad which has a reduced dimension rim on it's forward edge that sockets into the hollow stock. The stock interior has two molded in recessed lugs that the Phillips head screws holding the pad thread into. So, shortening the stock would require reshaping this rim and making new lugs for the retaining screws That doesn't seem practical.
Are you aware that the American compact has a shorter pull stock and Ruger may be able to supply one of these stocks as a replacement for your full size one?