Considering that the M77 Tang Safety is a push feed with a button ejector and a claw extractor, I wouldn't waste the money to open the bolt face for WSM. Similarly, the M77 in .270win would be a long action, while the 300wsm is quite comfortable in the short action for which it was designed.
Stick with a standard .473" bolt face, 3.340" long action cartridge. Nothing wrong with a .270win at all.
I do love the 338 A-square though...