First of all, be sure to measure the overall length of the axle to ensure a correct fit. And then...

Here are some of the rather confusing variables available on old Nervar crank arms which you should also be concerned with.

This is a detail of a 1974 catalog page which was scanned by enthusiast Robert S Broderick. By this point in time, cottered steel cranks were really on their way out in favor of a growing number of inexpensive alloy cotterless cranks. But, Nervar was still producing a wide variety of cranksets in their earlier patterns. With the variations shown in the image below, you could find a very inexpensive replacement crankset to match any of your existing bottom bracket or pedal thread needs. - But, that was then...

Best to accurately measure the diameter of the end of a BB axle. A 5/8" (British standard) axle = 15.88 mm. It will fit into a French diameter opening... but just a bit loosely. A 16 mm (French) axle will not fit into a British crank opening. Also, proper British dimensioned cotter pin [3/8"= 9.5 mm] will not fit into the smaller bore of a French crank arm. To be honest, I've never tested a mis-matched fit, but you might have an issue when combining the two sizes... the tapers may be a bit off and so with such a combination the cotter pin could prove too loose a fit to anchor the smaller axle properly... But, maybe it's no big deal at all.

In any case, Good Luck! Cottered axles are becoming very hard to find - just in a correct required length!