You are a lucky fella , I have hunted with several Mod 12's and can say , they don't make them like that anymore !
To answer your question , you could add pacers to the stock to fit you or you can buy a stock from Macan Stocks
to your dimensions , a good place to look also would be NULINE ind. or EBay.......The only thing you have to know is
the gauge . It's easy to replace as with 2 screws holding the recoil pad and inside the stock hole , there is a slot headed
bolt to unscrew with a long shank screw-driver .
With you being 6'3" a 15 1/2" length of stock would be what you need.......go to a Gun Store and ask them to measure your