Valad,
I approach this problem from a different angle. 20-gauge shotguns are popular because they are light and handy. Most of them kick like mules. Most men or boys put up with the recoil, especially if it means hauling around a lighter shotgun all day. Women and girls however, hate recoil and won't put up with it. The best shotgun for them fits perfectly and has some mass.
You're on the right track with your willingness to cut the stock and make things right in the "fit" department - fit is the key. But I would consider sticking with a 12-gauge. A moderate-weight 12-gauge kicks less firing AA shells than a lightweight 20-gauge does, and the shooter isn't handicapped by the pellet count.
An 870 Mag 12 with a 28" barrel is a beast, and I'm not surprised a Lady of your wife's dimensions would have trouble with it. I'd be careful though of not jumping too far in the other direction with a light 20-gauge.
A moderate-weight 12-gauge (maybe even a recoil-reducing semi), a stock that fits with a nice recoil pad, and AA shells might be just the ticket.
