It's all about chain line. The crankset manufacturer specified a particular spindle length. This length can usually be found on VeloBase if you know your crank. Adjusting chain line for 5-to-6 speed conversion should be minimal, at most 2.5mm outboard. I'd ignore that, personally. Note that your original spindle is asymmetrical, meaning that the drive side crank is placed farther away from the centerline than the non-drive side crank. This shouldn't matter if you choose to get a symmetrical cartridge style bearing, which I would also recommend. But you can shim the drive side of the cartridge to split the difference between the asymmetrical lengths.