I I know that the 308 series rifles use a custom lower that is machined bigger and thicker. I'd wager that the 300 SAUM uses the same. This is why they don't offer 308 uppers in thier catalog or on the website.
However I do know that a person can order complete 308 uppers from the factory. I suspect that if you already had one of the lowers from the 308 series, you could order a 300 SAUM upper.
It would probably be best if you called the factory directly.