What about having Canti/V brake studs brazed on?
Personally I don't get the whole 650b conversion thing, seems like it would be easier to just put slightly bigger chainwhels and slickish tires and on a quality MTB frame.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk