Try contacting this guy. He's already done what you're inquiring about. I don't know him, maybe he'll respond.

http://www.varminthunters.com/cgi-bin/classifieds/classifieds.cgi?session_key=&search_and_display_db_button=on&db_id=5612&query=retrieval

-Rick

One thing you'll have to be concerned with is wether or not there is enough shank left on the Winnie barrel for set back. The relatively mild load I use shoots 3650 with 53gr Hornady match bullets. I have had it to 3850 with a hot load of AA2520. My rifle wears a 26" tube so I would imagine 22" should still get you the 3500fps your looking for. If the setback is feasable then I'd imagine a properly cut chamber and a fresh crown should net you better accuracy then you previously had with the 22-250.