What the other guys said, mostly. The crank thing, I'm not sure about.

But if your chosen crankset bottoms out on the Stronglight spindle that
came with these originally, you can get a sealed unit BB square taper
that will work that has French thread cups from Velo Orange for about
40 bucks. If you're a big spender, Phil Wood has an infinite array.

Look at Sheldon Brown for both French tubing dimensions (some differences
even in the 531 frames ) and also read what he has to say about swapping
out square taper cranks and spindles between JIS and ISO.

http://sheldonbrown.com/bbtaper.html

There's a strong possibility that your chosen Shimano crank will be JIS and
won't work on a Stronglight spindle, but the Stronglight crank should work
fine with your other stuff if it's not worn out.