I got a good deal on a .375 Whitworth action. Is this action long/strong enough for a 416 Rigby build.

It is certainly strong enough, and can be made to function with the Rigby, although you will have some work to do. Many original Rigbys were built on standard M98 actions, re: Harry Selby's rifle. You will have to open up the bolt face, and the rails. I'm not certain if you will need to open the length of the action and magazine any more or not, but you should realize that there is going to be work and expense involved in this. Have you considered using a drop box magazine and bottom metal?

That said, it should be a "class act" when completed! Best of luck with it, and please fill us in on the details if you pursue this.

I disagree; two troubles here, lengthening and widening. The latter is the concern. It may work out, but there are other options out there that are much better, such as a factory Ruger M77 RSM or a CZ, or if you're really feeling flush a real Oberndorf Magnum action or a Brevex.

As Gwindrider1 said, the practical side of opening up a Mark X is a losing proposition. These were strong actions for standard sized cartridges (to .375 H&H diameter), but a little rough on finish (at least the couple I had were - I built a .458 on a new bbl'ed action in the lated '70s, and used a great deal of valve grinding compound to smooth it up).

I'd use the Mk X for something else...a 300 H&H comes to mind.

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.

.416 Rem would be better yet, as MD notes here. Maybe not the pinache of the Rigby, but about equal in performance.