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The 6v switch will be ok for the long term. The limiting factor isn’t voltage as much as current. Since the current will be lower at 12v it will be fine.
That said, running all that current through any switch is not the best way to go. I installed headlight relays in my 48, one for low and one for high beams. The current through the switch is just a few ma.
^This is great advice, too. The headlight relays take the heavy work away from the switch itself, which doesn't necessarily allow all of the current to go through to the lights anyway, and off-loading that to the relays often produces a brighter light. The Galaxie also got this upgrade over the summer. The difference in lighting was night and day (no pun intended).