Dave, here is BAD news…. when I worked at General Motors in 1971-73, I gave the engineers at Delco Remy a Lucas 12 volt coil from my 1969 Sunbeam Arrow. They tested it thoroughly and determined that although it was only 3 years old, it was not putting out 1/2 the voltage it was supposed to… that was with 12 volt input. The problem was deteriorated insulation inside the coil (on the windings). So, adding a resistor in the primary coil circuit was not a good idea. What I did was to use a coil for a 1969 Dodge Dart along with a resistor for that coil, I then used a relay to short out the resistor and wired the relay to the starter solenoid. Thus I got 12 volts to a 6 volt coil for starting. That car didn’t just start, it virtually exploded to life when you cranked it. It was by far the best starting British I had ever owned. Dave, by the way, that was the Arrow you were going to buy.