You need to first do some simple math. The difference between 26” mtb rim dia (559mm) and a 27.5/650b rim dia (584mm) is 25mm or 1”. Which means you need to raise you brake pads up 12.5mm, or 1/2”.
These are theoretical numbers and the brake surface diameter is not going to be the bead diameter, but you will need about 1/2” of space in the brake pad mounting groove. You might pick up a tiny bit more if you lengthen the groove slightly.
As others have said, the brake pad might contact the rim at too much of an angle if you are stretching the limits to reach the rim brake surface. A narrow rim “might” help to square this up a bit, I’m not sure. But I would think finding a 14mm or 17mm wide 650b/27.5 rim might be a little tough.
Someone with more experience in vintage 650b rims can offer a better perspective. But I doubt modern 27.5 rims are that narrow.
John