My Holdsworth 650b conversion is really tight with 42mm tires (Hetres) on Weinmann ZAC19 rims, especially so since the tires don't seat well.
Really tight, like it varies between 1 and 3 mm over the course of a wheel revolution. Potentially a real problem, but I've ridden it a couple thousand miles without an issue.
Nonetheless, on long rides (i did a fleche and an arrow this year, a 300k last year, &c) I have carried a 38 mm tire (just in case) and obviously I'd have a little less to worry about if I had a little more clearance.
Hmmm, with that knowledge I think the vertical dropouts you sent are probably out of the question...
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