Dear Mike,
If I can take a #3 in 45-70 and make it into a 17 Ackley Bee [Rimmed case too] you can do the same with the 256 Win Mag! I took off the 45-70 barrel late one night and put on a Rem 700 17 Rem barrel. Chambered it out to 17 Ack Bee and reblued it. Shoots great!. You can get different extractors from Ruger [Brownells too] for cheap and all you gotta do is figure the closest match. I would think one for a 30-30 would work. The most difficult thing you might encounter is figureing out the scope blocks for the barrel. A little trial and error will work there too. Go ahead with the idea! I think it`d be a neat project.
Aloha, Mark