I think the only cures for the large cylinder gap would be, removing material from the end of the arbor, and a like amount from the barrel lug. And then re-fitting the wedge. But I doubt you'd want to perform such modifications to a valuable original gun.
You could also have metal added to the forcing cone by having it TIG welded. But again, major mods to your original gun.
Other than that, I don't know how else to close that gap. Hopefully 45Dragoon will weigh in and give us his opinion. He's the true expert.