I'm restoring/updating an 1970 Fresh Tandem that has a 26" wheel (700D x 1.40) and I'm thinking about swapping the wheel set with something a little more modern and accessible like a 27.5" 650B because the frame clears it, provides a little more ride comfort and it looks nice on the bike. However, I'm a little concerned about the weight limit, we're a 300 pound team (135kg) and the bike weighs around 40 pounds. Am I making a mistake here? Here's a quick photo with both wheels on for testing. The old rims are 50 years old, the tires are extreemly difficult to find and I'm planning on swapping the freewheel with a 9-speed cassette anyway.
The rim size is immaterial to the strength of the set you choose.
Many rim brake 650b rims out there would not make good tandem rims, but there are exceptions. The VO Voyager or Diagonale in 36h are the main ones I can think of. Sun CR-18s are another, cheaper possibility. Also Velocity Cliffhanger or Atlas.
650b is likely the right choice for a conversion. If you happen to be maxing out your brake reach at present that could be a curveball, but at a radius difference of 1.5mm it's probably fine.