Can't help with the width but i support the advise to lace your B17. I had one that was stretched too much, and then the bolt failed. I ended up disassembling it, replacing the bolt, butchered, laced and mounted the leather top back using screws and nuts. And I ended up with something I was looking for a while, it's like a wider Swallow. The top part is pretty flat, it's as stiff as B17N (i have that one too) and i really like the width of the regular B17.

It's got a bit wider in the middle, and I can put it further backwards, which helps when i want to sit lower -- i don't need to sit right on top of the metal frame, it's easier to find the sweet spot. I guess it could help you a bit when mountain biking, it is easier to get behind a low saddle. In general i see a pattern with Brooks saddles: the more upright the riders position is, the more flat is the saddle. And it needs to be wider too to support the sitbones (well, sorry for saying the obvious).

Now it is my favorite saddle, I am glad I had a bit of spare time to torture it and did not throw it away.

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