I know that you gave some good weightlifting advice on another thread and probably aren't mistaken for Skeletor as often as I am, but there is a wider range of options in the WM bags than you think.
The WM Summerlite is a narrow mummy bag and has a shoulder girth of 60". You're right to note that some hikers find this to be too narrow for comfort. However, the WM Megalite is essentially the same bag, but with a 65" shoulder girth. The same principle applies to the Ultralite and the Alpinlite: both are rated to 20*F, but the Ultralite has a shoulder girth of 60" whereas the Alpinlite has a shoulder girth of 65". Both of the wider bags weigh a little more than their narrower counterparts (but still weigh less than an equivalent Montbell UL SS bag).
So, unless I (or any other tall and relatively slender hikers) join a major league baseball team and experience an unnatural growth spurt, I think WM has us covered.
That said, Monbell does claim that their UL Super Stretch bags expand to shoulder girths up to 75" and, therefore, can theoretically fit bulkier hulks such as yourself or anyone who is too girthy to live with a 65" shoulder girth. But what they don't tell you is how much you compress their bags' loft to achieve that sort of fit (answer: a lot). In practice though, if you need that much girth out of your Montbell bag, you're going to significanlty impact the temperatures at which you'll be comfortable and you'd be better off with another bag anyway.
I'm going to go ahead and say it, although I think it'll be a pretty unpopular statement: I think that the Montbell UL Super Stretch design is a silly gimmick.