I have a 1981 Schwinn Voyageur 11.8 built in Japan by Panasonic. It takes a .833" quill stem and originally had a BMX type headset. I recently replaced the headset and can confirm that the crown race is 26.4mm.

Is the original BB damaged or excessively worn? Chainline should be fine with the existing spindle so if it's in ok shape, just reuse it.