Re: Hello I am new to this forum but not to firearms.
Split the diffrence and go with a .300 Win mag. It has more case capacity than the .300 WSM as since you have the long action, that would make more sense. And you'd still have a repeater. There really is nothing a .300 WSM can do better than a .300 Win Mag in all practicality, especially in a long action.
If you want another 7mm Mag, get a faster twist bbl so you can shoot the heavier bullets with the Higher B.C. They fly better and hit harder at longer distances.
I'm not Familiar with McGowen bbls, But I have shot Shilens before and I have a Shilen that's going to be a 6.5-06 AI really soon. So I say Shilen. Just make sure you have at least a 26" bbl.