I'm late to the conversation, but I've done a bunch of 650B conversions over the past 6-7 years. My first was actually my most ambitious and most successful-- I'm still riding that one regularly. It's a 1984 Shogun tourer made for 27" wheels, and had a very high bottom bracket, handily lending itself to conversion. I had the usual braze-ons added (extra eyelets, bottle mounts, and canti posts, and provisions for internal wiring), had the whole thing powder coated in cream, and built it up as a 650B randonneur-style bike. It fits 42mm wide 650B tires with very little room to spare:



The bike wasn't perfect for this setup and didn't do well for long rides with a front load, so I reconfigured it as an around-town porteur-style bike, and the difference in riding position transformed the feel of the bike. It works really well in this configuration and I've been using it in this form for a few years:



I built this new Soma Buena Vista frameset up for my wife using 650B wheels right from the start (no modifications necessary... gobs of tire clearance for 42mm tires... just needed long-reach brakes), and it has been her errand bike for the past six years. It has a low bottom bracket, so I might not recommend this frame for 650B road riding unless you were to go with shorter cranks. It's had a few changes and swap-outs since then, but it still retains its 650B format:



Since then I've added 650B purpose built frames to my fleet (Rawland Stag and a Jeff Lyon L'Avecaise custom), but haven't played around with more conversions.