I have a Shikari, too, with just the .45-70 barrel. At the time the gun was manufactured, they high-temperature brazed the barrel lugs; they are now what appears to be electron beam welded.
The Shikari is safe for Springfield loads. The 20 ga barrel will probably fit the newer actions, and for shotgun pressures, would probably be OK. Don't use the .45-70 barrel on the newer actions........it will just give a false sense of security; the lug attachment is just not as strong.