When wires melt, you have a ground (read short circuit) that you shouldn't have. And since it happens when you turn on the ignition, it sounds like the short circuit is somewhere in the ignition circuit. What wires melt/burn/smoke between the +ve terminal of your battery and the short circuit will be partly determined by the gauges of the wire in the circuit. The stock 6v wiring should be more than adequate because current requirements were higher with 6v.