Ruger won't sell those barrels separately. The reason is the way the guns are made. They take a piece of heavy wall steel tubing, inside thread it, screw in the finished barrel, then use the feed ramp of the barrel as a reference point to cut the ejection port, drill for the ejector and bolt stop pin, etc. So each barrel and receiver assembly is made as a unit and it would be almost impossible to replace the barrel and have it fit right.
Incidentally, the Standard Model is not the Mk I. The Mk I is the first target version, made with adjustable sights and either a bull barrel or a 6 7/8" barrel.