Valad,
I can't be much help in finding a used 870 stock. I don't think I've ever seen an 870 action that wore out before the stock did, and guys tend to hang onto extras for the same reasons you're looking for one. The new stocks I've seen go for around $80.
If the Super Mag is the way you want to go, I'd cut the stock you have and worry about it later - there's always the chance that your wife will like the set-up and you won't need to replace it anyway. That wouldn't be my advice on a high-dollar shotgun, but an 870 kinda is what it is.
I do think the 28" barrel is going to be a problem though, and my wife would probably tire of the whole routine of new stock...test, shorter barrel...test, different load...test - especially if she is getting whacked by the shotgun the whole time. I'd want to solve the problem in one swoop to minimize the Crabby Index.
Cutting the barrel is more of a problem money-wise, especially if you're considering screw-in chokes. The nickels and dimes are starting to add up with a new stock and barrel modifications, and a Super Mag with a short barrel is almost sure to be a white elephant if you ever try to sell it.
All in all, your Super Mag might be convenient choice because you already have one, but it might not be the best choice in the long run.
