It sounds like a mid-1970s TdF. I always considered the Stronglight crankset to be the best component of the bicycle, so I find it strange that you want to swap it out. Regardless, if it has the original Stronglight, eight sided, fixed cup, you can easily determine the threading. A French cup (most likely) will have a single ring cut into the face. A Swiss cup (less likely) will have no ring. An English cup (even less likely) will have two rings. I'm assuming it wasn't stripped and tapped out to Italian (least likely).
Regarding the bottom bracket, I'd install both cranksets on the old bottom and measure the distance from the seat tube to the inner chainring. This will give you the difference in the profile of the cranksets. Now measure the old spindle protrusion from the face the bottom bracket shell on the drive side, as the old spindles were asymmetrical. Double the protrusion measurement and add it to the width of the bottom bracket shell (which should be 68mm) to determine the equivalent length a modern, symmetrical spindle. Finally, correct the length by adding or subtracting double the difference of the crankarm profiles measurement. This spindle length should duplicate the old chain line, provided the modern tapers are the same, as suggested by previous posts.
Personally, if I was converting the cranks to 6400, I'd convert the whole bicycle and not hesitate at including the brakes. While the MAFAC were good performers in their day, IMO, 600 Ultegra easily outperforms them. The main hurdle will be finding a set with mounting bolts for exposed nuts.
Last edited by T-Mar; 08-25-16 at 09:09 AM.