initially id start with something less permanent than filling the stock with lead/epoxy.
if later on you realise that the balance of the rifle is all f'd up and its too heavy, you might want to undo it - and thats not an option if you use epoxy. maybe its just me but if ill always take the completely reversible solution over the permanent, irreversible one.
sew up a beanbag full of lead shot, drop it in the stock hollow, squish it in, and then cut a slightly oversize plug of high density styrofoam insulation and jam it in behind it, screw in the buttplate, it wont move.
ive done this to my friend's Stevens and it hasnt budged in 2 years now, but if he ever changes his mind or wants to sell the gun or something he can remove it in seconds.