I personally don't think that your going to get a straight up trade. The .357Mag barrels are hard to come by. It took me a year to find one. But it was worth the wait.
As far as the 45-70 barrels go, they occasionally appear for sale but are usually snapped up quickly. I think you would be better off selling the .243 barrel, and then purchase one on this site, when it becomes available, or go the factory route.
If you want, post an ad in the WTT, and see what happens. You never know, you might get lucky.