I'm just wondering if anyone can give me a little help with my 70's crossbike. It currently has Nervar 50.4BCD cranks with a single 45T chainring and an old Campag Record BB. I have a set of TA 50T outer & 40T inner chainrings that I would like to use to make it a double, but I'm a little unclear on what bottom bracket to use. Velobase has a Nervar catalogue from 1974 that says 117mm axel for a double, and Sheldon Brown has Nervar as needing an ISO rather than JIS type BB. What options do I have, if any ? Any and all help would be greatly appreciated.

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.

What chainline do you have now? Measure that first.
Are you on the inner or outer ramps?
Is yours a pre or post CPSC spindle? They are different lengths.

You want a 43.5mm chainline. That equates to a 41mm inner, 46mm outer. Lets assume you are now running 44mm on the outside ramp, and you have a 112mm pre-CPSC "Campy pre'94 taper" spindle.

To find an equivalent Stronglight/ISO spindle, you would take your 112mm, add 3mm to make up for the ISO/old Campy difference (that number is from my own experiments), then add double the difference from your desired chainline to your existing chainline (46mm-44mm)*2 = 4mm for a total of 119mm.

You also need to find one that has the same drive-side offset as your existing spindle, or adjust for that as well. IIRC the Campy's are +2mm DS, and so are the Stronglight 118.5mm doubles.

Also realize that close is good enough in the old non-indexed world. Don't sweat it if the closest spindle you can come up with is 3mm different than spec. Divided by 2 that's 1.5mm per side, and I'll bet your components would handle it OK.

I used a 118mm Stronglight spindle and needed to use a 1mm spacer on the drive side so the chainring bolts would clear the chainstay...That's my experience.

Thanks guys, exactly the help I needed. Will report back when I get it all sorted out.