You can use a JIS spindle if you need to but it will move your chainline out a few mm. Otherwise, just mount the 40/50 rings using your current spindle, calculate how much you want to move the chainline and find a spindle or BB to fit. Pay attention to the distance between the ridges of the spindle since thin cups vs thick cups can be different. If $ isn't a concern, you can get something like a Phil BB that has adjustable cups on both sides to allow you fine tune the chainline if needed. I think these are available in both JIS and ISO.

Edit to add: I've heard the Nervar 50.4 used the same BB as Stronglight and TA which were usually 118mm/ISO for a double. Those spindles show up quite frequently on ebay.