You can either buy the 12 volt coil (with the internal resistor) or use the old 6 volt coil and add the external resistor.
As far as checking the voltage was your points fully closed? You have to check the voltage with the points fully closed or you will get higher voltage as there is no current flow and resistors only drop voltage if current is flowing. But with the points closed you should get about 8 to 10 volts at the + side of the coil. (make sure the ignition is on too)
I've never tried running the engine and checking with a multimeter but to me that sounds like it would not work as now you get a form of alternating (more puseating than true AC but anyway)current which will give be at 6 to 10 volts for one split second and jump to 12 to 14 the next.I have hooked a single cylinder ignition system to a oscilloscope though and could find the pulse and measure it but it the pulse of point contact was hard to see. But with a 6 cylinder it would be more frequent may be detectable with the right DMM. But points closed is what your looking for.
