You'll get a pretty healthy flash from the booster cable as you attach the last connection because of the mismatch in battery voltage. I always make the ground cable connection last, as far away from the battery as possible to prevent any danger of getting a hydrogen gas explosion. Don't crank with the 12 volts attached for more than 10 seconds or so to prevent any damage to the coil, points, etc. Disconnect the 12V jumper as soon as the engine starts.
One thing I've done while waking up a 6V engine after a long sleep is to install a 12V coil temporarily and use a 12V battery to do the initial startup, run-in and tune up. Just disconnect the Battery wire at the voltage regulator to prevent doing any damage to the charging system, and leave the lights and other 6V accessories turned off. Once it's starting and running reliably, you can swap back to 6V operation.
