Forgive me if you already know this, and I don't claim to be an expert, but I think you need to sort out a couple things.
A rifle's magazine mates to bottom metal that screws into the action (and traps the stock between the action and the bottom metal).
As I understand it, aftermarket magazines and bottom metal like the Hawkins are for Remington 700 pattern actions. Do you they make bottom metal and magazines for the X-bolt? If they do, I'm unaware of that.
You mentioned replacing the stock on your X-bolt with a McMillian, etc. The McMillian will need to be ordered with a specific inletting not just for the action, but also for the bottom metal. Or possibly modified by the user or a gunsmith. Can you get a McMillian stock inlet for an X-bolt? And if so, is it inlet for factory Browning X-bolt bottom metal and magazine?
There are some Browning X-bolt models that come with a McMillian stock from the factory. And they have the X-bolt bottom metal and magazine. That would be awesome if you could get one of those...but is it possible?
Could you use your existing stock? Might not be easy to do if you are going with a thicker profile barrel.
Now if you modify and use your existing stock, or you somehow get McMillian stock and can use your factory bottom metal (or for all I know the bottom metal on factory X-bolts that come with a McMillian stock might not be exactly the same as your bottom metal).
But if somehow you can use a factory X-bolt magazine, I'd look at cases that also use the 404 Jeffrey case. 300WSM is a shortened, rebated rim version of the 404 Jeffrey as I understand it. Other cases that share the 404 Jeffrey parent case are 26 Nosler, 28 Nosler, etc. So I would think that a 300WSM would feed decently well out of the 28 Nosler magazine, but I could be wrong.
Hopefully that helps bring some things up to consider.