Well in my view your parts dealer is talking a load of bollix.
First of all nearly every car of that era uses 12 volts. I'd be MOST surprised if your alternator was 6 volts. I remember 6 volts back in the 1950s but by 1970, no!
You can confirm that by looking a any bulb, it has the voltage on it.
But back in the 70s it was quite common to have a ballast resistor in the supply to the coil, which dropped the voltage to about 9 volts. The coil was specifically designed to work of 9 volts. However when you tried to start the car the ballast resistor was bypassed and the full battery voltage applied across the coil. Since the battery voltage will drop while cranking the engine in made sure you got a decent spark.
I would also be surprised if you can get a Spitfire wiring loom, so I would go for a kit car loom. While you are at it upgrade the dizzy to contactless and use an after market ignition system.
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