Actually, since you will probably be handloading your ammunition, there would be no reason to correct the headspace, since the .375 Weatherby has enough shoulder use it to to headspace on. If you fire form your brass with cast bullets seated out to touch the lands, using a mild load of propellant, the formed cases will be perfectly headspaced for your rifle, and you can load them with full power loads with no fear of case separation, as long as you don't set the shoulder back in the resizing process.

Headspacing on the belt makes no sense at all with a case formed with a steep Weatherby shoulder.