when you measure different 10,75x68 cartridges and bullets you will find that the cylindrical 10,75 mm part of the bullet will end on the case mouth. the ogival part of the bullet will reach into the rifling but dont touch it so you dont have problem's to load it into your gun.
however the 10,75x68 have a free bore when the chamber is cut with the correct tool and I would give the rifle to someone with such a chamber reamer. its not original anymore so don't risk anything.