Chamber lengths are set by SAAMI. Drawings and specs are on their site.
A new barrel will have nothing to do with the old one anyway.
However, since the .260 Rem is just a necked down .308, the chamber dimensions should be the same.
Lotta stuff like the Hornady OAL kit, which is supposed to be about seating depth, is more about separating you from your money than any practical value.