Octalink v2 bottom bracket on italian shell
The spindle length depends on crank and chainline (distance from center of top tube to chainrings). In road bikes the chainline is almost always around 43 mm (exception being boost spacing), so the safest bet is to pick the same spindle length as on the bike where the cranks came from.

Different cranks may have different chainline on the same spindle.

If you can reassemble the original bikes, you should be able to measure the chainline on both of them so that you can see if you need to change the spindle length.