I once had the autographs of the Denver Broncos from 1989, including John Elway, that my sister got by accidental meeting in London of all places. She, another sister of mine, and a future brother-in-law got to hang out with them in London a few nights. One day, I gave the slip of paper on which the signatures were affixed to a girl I loved and was sure to marry. I haven't seen that paper or the girl for more than a decade.
A Browning Superposed is a very nice shotgun. While it does need to be used - firearms do need that - at what point do you willingly permanently alter a firearm for someone who may or may not even like shooting it after the fact? Or, someone who may not be a keeper? Mine was a keeper. Man, was she a keeper! And she kept my Denver Broncos (she WAS a fiance). I say not only no, but HECK no. Wait for the ring at the very least, and by that I mean wedding ring. A month away from fiance is still a girlfriend. Women folk are a fickle breed of creature on which few lasting predictions may be made.
Nah, forget that, don't do it at all. That shotgun deserves to be used, and if it gets cut down and never gets used at all, what was the point? If you can't use it, if she doesn't use it, it is now a waste of materials that cannot even bring a good price. Get that replacement stock and have it cut down (or perhaps see if a shorter LOP version can be found). You can even have it laser-etched with something personal for her. If she likes it, she'll cherish it. If she doesn't, well, put the original stock back on and it's yours.
The shotgun is yours, true, but it was also your uncle's. Preserve it as a family piece. I see folks sporterize military guns and claim that it is theirs and they can do to it what ever they want. While they are correct, I support private property rights, guns that get so treated rarely get used. The sad fact is that they end up collecting dust in pawn shops only to be bought for parts down the road. They are like a pretty girl going into cheap porn. Ten years down the road she is a worn-out hag doing meth.
That ain't no future for a Browning Superposed.