I would ignore the old manuals, particularly if the loads exceed the current data. Many old manuals were published in an era before actual pressure testing was the norm, and loads were determined by the gross vagaries of pressure signs apparent on loads fired in production guns that could, at times, be atypical of most other guns out there. Modern load development is done in SAAMI spec pressure and velocity guns that have tight tolerance minimum chambers in order to present a worst-case pressure situation. That is a much more reliable approach.
Stick powders can last a long time if they are kept at cool or modest temperatures. If you sealed them in plastic and put them in your freezer, they would last a good number of centuries. The British military limits them to 45 years in stockpile storage (loaded ammo kept in cool-ish bunker temperatures, but still safe to issue), while they limit spherical propellants to 20 years in those same conditions. The latter suffer breakdown faster. In typical basement environs, stick powders can go well beyond that 45-year time frame. 60 to 100 years would not be outside the realm of possibility.