I think you're better off sticking with 26" wheels. People switch from 700c to 650B to get more tire clearance. Going from 26" to 650B will have the opposite effect. That said, if you really want to do it (and I've been known to do things like that even after everyone tells me it's foolish), this is the product you need:
https://www.universalcycles.com/shop...s.php?id=67799
Edit: that will only work if your canti studs are removable. There's a very good chance they aren't.