Not much difference in the case other than neck length.
If OAL in within limits it should work in the spool magazine.
Standard 150 gr hunting loads for the .308 are not high pressure loads. Heavy bullet and long range match loads, along with some 7.62 NATO long range MG or sniper loads may be too hot for the 99.
The savage lever action was at one time available in .308, though this version used a detachable magazine instead of the more common spool type rotary mag.
I'd strongly advise against rebarreling or rechambering a .300 Savage model 99 if it has a good bore and chamber as is. The unaltered rifle is a collectors item these days.
Last edited by Rainbow Demon; March 12, 2013 at 10:39 PM.