Depends on how badly you want it, but you could find a 6v to 12v transformer, hide it under the dash and run a wire up to the motor. If you're really good you can make your own. Just make 2 coils of copper wire, one with twice as many coils. Maybe a more electrically minded poster can chime in on exactly how.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.