Finally got around to Phase Two on the Trek 400 -- converting it to 650B. I mounted the Panaracer Col de la Vie tires and took it for a 30-mile shakedown mixter loop through the Marin Headlands on Monday evening. The bike is a lot more fun with these wheels/tires! Still need to work out some fit issues, though -- I think I need a shorter stem.
On a semi-related note, I discovered that 9-speed Shimano shifters index quite well with an 8-speed cassette (the one I lost, then found, which ironically enough, I'm not using on this bike now).