The 8x68S was apparently designed to fit into a standard Mauser M98 action.
Magazine length of the standard German M98 action is ca. 84 mm. One of mine, mod. 1909 for Argentine, measured 83.6 mm.
Modern CIP L6 / max. cartridge length of the 8x68S is 87 mm.

I have read that the magazine length of late commercial Belgian FN actions is a bit longer, around 86 mm, but never measured one of these. Also can't say what "late" means. Anyway, this will fit just fine for the 8x68S.

Length of a Mauser magnum magazine is around 96 mm.
This action was originally used for the Rigby cartridges.
CIP L6 for the .350 Rigby Mag. is 90.8 mm.
I do not know the dimensions of factory ammunition. You have to do some measuring and then decide.
With handloading this will be no problem, as you can adjust the cartridge length by yourself.

If the bore of this rifle is still good, and if it has the correct twist length for the 8x68S (280 mm instead of the normal 240 mm for 8x57IS etc.), then I would happily keep it as it is!

Fuhrmann