Sourdough,
I know those Night Force scopes are big and heavy, and that 338/378 is a stomper. The "perfect storm" for mounting issues.
I don't know if a T'SOB mount is made for your rifle, but I'm thinking it might be.
I'd try SSK for the mount and you can add rings. Another suggestion might be to go to larger diameter mount screws as well, if you haven't already.
http://www.sskindustries.com/rifle.htmI've talked to J.D. Jones before when I had some Encore work done. He was very helpful. I'm sure he would have some good advice. I hope this helps. CM