Given the shape of the chain stays I think you wouldn't win any clearance with 650b.
FIY, I've been able to fit 650x42b grand bois hetre tires on my pompetamine, stretched to 44mm on dt m442 rims - I get ~3mm of clearance on both side on the rear. I'm quite sure that you can not fit 700x42 on that frame :)
@guillaume started
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Given the shape of the chain stays I think you wouldn't win any clearance with 650b.