Worry not about the strength of the Model 99's action... it will take a "hot" load in a .308 Win. with no problem at all.
The breach-block in a Model 99 Savage is massive and the whole rifle is very strong. I've loaded many a "hot round" in my Model 99 in .300 Savage caliber (the "father" of the .308 Winchester cartridge).
However, generally, I've found the MOST ACCURATE loads in ANY caliber of rifle are usually slightly less than the maximum listed loads in most cases.
Strength & Honor...
Ron T.