The primary reason for the change to 12V systems was to cut down the size and weight of the wiring (same reason some lux. cars are now going to 24V). More voltage, less amperage required, hence smaller wires needed to power a given component.
That said, there is no cause for concern in converting an older 6V car to 12V, as the original larger gauge wire in those 6V volt systems will be loafing when asked to pass just half the amperage of a 6V system.
I've switched my '40 60 Special over to 12V, including replacing the generator (and regulator) with a modern and inexpensive self-regulating alternator. I now have plenty of power for my added air conditioner and electric cooling fan. Be happy to pass along my installation notes and updated wiring diagram if anyone is considering a similar upgrade.
If your primary concern is the fact that your generator is working overtime to keep up with the demands of an A/C system, you might want to consider installing a 6V alternator.. these are widely available, even in Positive Ground versions, and include voltage regulators, so you can eliminate another source of problems in your original system. (And, it would probably be cheaper than buying a relatively rare 8V battery, and re-adjusting your regulator)