Personally, I'd go with your Plan B, which is exactly what I did with my '74 Cinelli SC. That said, it's your bike so do what you want...very carefully. You seem to be assuming that the OLD is 126mm. However, that seems unlikely. My Cinelli is 120mm; my understanding is that for most bike brands 126mm didn't come into common use until the late '70s and early '80s.