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.
