Stan takes in the twins when their parents find they can’t provide the upbringing Dipper and Mabel need given their new circumstances. The Mystery Shack is converted into the first ever Library on the Supernatural, though Stan does still sell a few knick-knacks on the side. Old habits and all that. He lives a long, fulfilling life, passing away well into old age and long after getting to meet not only his great-great niblings, but even some great-great-great ones.
Ford…Ford is where it gets interesting.
You see, this AU began before Ford returned to Gravity Falls, and so a lot of the establishing material didn’t include him. A lot of the establishing material, for that matter, still trusted that Stanford was Stanley’s real name, and that therefore Stanley was the then-theorized lost twin brother. This is why this AU has the Stanley Pines Memorial Library – as briefly mentioned above – rather than one for Stanford.
Later material is therefore split along at least a couple of lines: some continue to write that era of the AU as if Ford never came back. Weirdmageddon went differently; the portal wasn’t fixed or activated. Others say that yes, Ford returned, and most of the show continued as normal with the obvious and AU-necessitated change in that Dipper was more directly involved in destroying Bill and became a demon in the process. In some of these fics, Ford is mostly present in the Shack-turned-Library. In others, he treats the Library as a home base, only popping in now and again while he spends most of his time out in the world, researching and documenting in the field, so to speak. It depends a lot on the individual writer and how much they want to change from the way the AU originally went.