When using the saw, use a fine blade and make sure to rest the gun on something so the cut will be straight across the stock (as viewed from the top). Remove the original buttplate first so you don't cut into the screws (scrreeeeeecccchhh!!!), then make the cut parallel to the same line. If you want, you can then fit a recoil pad or put the original buttplate back on.
To do that, screw the pad or plate to the butt, wrap masking tape around the stock just ahead of the buttplate/pad and use a belt sander to trim the pad or plate down following the stock lines. The masking tape will prevent a small mistake from being a big one. At the toe (bottom of the stock) be sure to cut the plate or pad in line with the stock line the way the original was.
Jim