Thanks to all the helpful tips. This is a roadbike, not a mtb, I'm not even sure that was much of a thing in 1978, much less a motobecane thing. I was able to secure the correct size seatpost today. It's a 26.0 for those counting. It seems all is well now, and this should be the resolution. Gotta give props to DannoXYZ, who had the most practical tips. Hard to believe the shop would steer me wrong by 0.4 mm.

Either way I made out pretty well as I was able to pick up the correct seat post and found a new bottom bracket which solves all my Swiss BB problems and for only $2!