Check the condition of the points. Most likely, you'll find them burned from carrying the excessive current the 6V coil allows when it's running on 12 volts. Temporarily, you can clean up the points with a fine-cut file or some sandpaper, but they'll need to be replaced before you hit the road. An ignition resistor for any late 50's/early 60's GM vehicle will work with the 12V coil to get the point current right.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!