This may not be the problem your experiencing but try taking out the ballast resistor wire in the ignition circuit. That will up the voltage to the coil to a full 12 volts instead of the 9 or so its getting now. The pink resistor wire is under the dash and leads from the ignition key( actually about 10 inches of copper wire -then the pink wire) to the plug(it goes all the way up to the plug on the interior under the dash) at the firewall just above the master cylinder. You can replace the resistor wire with copper wire. I think the whole system runs better at full voltage.  

I put one in my 70 M-1 back in 95 and never look back.  COme to think of it i havent changed the plugs since then either, 10,000 or so miles driven.  no prob what so ever