That sounds like a fun, exciting, and ambitious project. I guess one of your pair will be chambered in .416 Ruger, but what will the other one be? Are you going to put iron sights on or just go with slick barrels? How many recoil lugs and how many cross bolts?

It is really smart to start with a commercial action like the M77 instead of the way I started with old Mauser types of actions that had to be modified. Those older rifles that I had were ultimately expensive and really too heavy.

I do have a stainless Mark II chambered in .338 that I do not use very much, but was concerned about feeding issues if it was rebarreled to .416. Maybe I will learn something useful from your project.