I just got a used Shimano RX100 crankset, and was planning on putting it on a '72 Peugeot UO-18 mixte, using the VO bb: http://www.velo-orange.com/grcrufrthbob.html

At least that was the plan. Someone just told me however that the RX100 may be ISO, rather than JIS ... what!? I assumed (no need to define that for me) Japanese cranks use the Japanese standard? The RX100 is off a 7-spd (1990-3). I've also got a Shimano 105 (biopace!) from '86 that I could use. I notice Sheldon's chart doesn't even spec the 105 back that far. I'd rather use the RX100 however.

Next question - spindle length?
I have to spec this to order the bb. Here's what I know so far:
The bb shell on the peugeot is 68mm.
I'm spreading the rear dropouts to accommodate a 126mm hub, with a 6-spd freewheel from late 80s.
The VO spindle is symmetrical, the current (cottered) is asymmetrical.

Any tips how to determine spindle length?

Another way to ask this - should I shoot for the same chainline (center of seat tube to large chainring) as I had with the cottered crank? If so, that would put the non-driveside crankarm further away from the chainstay. I assumed this would happen when I learned the VO had a symmetrical spindle.
I'll also mount the rear wheel (soon) and try to determine chainline from that perspective, although at the moment Sheldon's instructions for that make very little sense to me.