To take a standard long action and open up the port to allow for the extraction of a 375 H&H cartridge, how much of the port needs to be milled. Is this a difficult and expensive task? Is there a pressure issue with the small shank and larger calibers?

Since the magazine would need to be replaced, is there an aftermarket magazine that can be purchased also?

Case in point, finding an old 7 mag or 30-06 gun and making it into an STW, RUM, 375 H&H size action so as to be able to extract loaded ammo?