is this motobecane the one that is similar to the peugeot px-10 with nervex lugs and reynolds 531? just curious???

Some other things to consider. Once you find a bb that will work such as the velo, then you dont have to worry about locating cranks since it is square tapered...additionally the pedal problem falls off the radar. If you are intending on doing the cottered cranks/ french pedal threading, etc.. then its gonna get nasty.
Keep in mind that your headset will need to be french and that the stem/bars for this was the AVA branded murder stem that would break/snap apart. you can however use a 22.2mm quill stem and wrap a peice of sand paper around it to get it to 22mm steerer.

...oh.. i just reread the OP.. i dont know if all of what i said applies to mid 80s motobecane???