The easiest solution is to use the original French cups. Bearings are no big deal, the good old standard loose balls will work just fine. To this add a '5' (Italian) dimensioned spindle, since French cups are usually a bit thinner. Now, you can probably do all the math on chainlines, etc., to figure out the proper spindle length, but it's still an arcane science, so the easiest solution is to pick up a couple of different used spindles and just try them to find which one fits the best for your planned crank.

If you're planning on using a French crank (say, Stronglight), it might be a bit more direct to find an actual Stronglight, etc. spindle in the proper size. On my end, the local LUsedBS sells me spindles at $1.00 apiece, so I just have a collection of them on the shelf.