No problem. To mount them, look at the back of your shotty, about where the flap of skin between your thumb and index finger lays on the pistol grip. There should be a fairly large screw there. Take that out and the pistol grip should come right off. Put the replacement stock/grip combo in its place and replace the screw with the one provided with the stock.
IF IT DOES NOT FIT (different threading), then look at the length of the screw that comes with the new stock. If it's the same as the original one, then just use that. If it's longer (eg, the thumb-over full stock will have a much longer screw), then go down to your local hardware store and find a bolt of the same threading as the original one, and of the length (or longer and cut it down) as the new one.
Easy as pie, since you don't have a gas system to deal with
