I have a nice Pre-War 99R in 250 that could become a 358.
No, it couldn't.
Need one with serial number in the 900,000's or higher. Internals aren't right on lower serial numbers to handle the 243/308/358 length cartridges.
“The Savage 99 Pocket Reference”. All models and variations of 1895’s, 1899’s and 99’s covered. Also dates, checkering, engraving.. Find at www.savagelevers.com