Your Mark X action is perfect for the .416 Remington Magnum as it is. I know this because I built a .416 Remington on a Mark X 20 years ago, and it worked perfectly without any modifications.
I know Harry Selby's. 416 Rigby was built on a standard 98 action (in fact a military 98) because I have shot that rifle myself, and it worked perfectly for thousands of rounds. But there's no reason to spend the money making a Mark X work when there are better options today.
