Depending on the action, it is entirely possible to rebarrel either caliber to .375. The degree of effort and expense would depend on what you started with. Both cartridges you mention have the correct head size, so no bolt face alteration would be needed, but the .338 Win. is a standard length cartridge, and would potentially need the magazine, follower, and action lengthened to accommodate the longer H&H round. The Weatherby cartridge is only .025 shorter than the H&H, and should be an easier candidate for the conversion.
Hope this helps!
