Thanks, so it been done before, I'm not crazy for thinking of going 590 to 559 then.

From memory the front brakes on our big ugly Columbia have like 30mm (!) of adjustment range, and they're near the top with the 590 rim. I'll go caliper if the cantis dont work out. And hopefully a 559-38 street tire won't be too wide for the fork! I'll post a pic after.