Sir- I've only collected Ross rifles for fifty-odd years, but can safely say that the Ross M-10 only left the factory chambered in .280 Ross. Many have been rebarrelled later, though. If it's originally a .303, it likely started life as a MkIII Military rifle. Fitting Lyman blocks would be no problem at all, as long as you aren't mutilating an original rifle. If Bubba's already messed with it, you have nothing to lose.