Are you using a points ignition or a HEI electronic ignition? Here is some info I dug up for you. It is critical to understand that the HEI requires 12 volts AT ALL TIMES to work at it’s best, and points would burn up with 12 volts at all times, so the factory added either a inline ballast resistor or a resistance type wire that reduces the 12 volts from the battery down to around 9.3 volts to the coil. The points-type distributors do use 12 volts when cranking, however, this is accomplished with a 'bypass wire', located between the starter solenoid (R) terminal and the coil (+) terminal, and is 'hot' only in the ignition switch's 'crank' position and "bypasses" the resistor wire to supply the 12 volts. In other words, on some cars, there will be 2 wires for spark juice, one is 'hot' in the ignition switch "crank" position, the other is 'hot' in the ignition switch "run" position. This was common in the pre-1971 cars, which used an inline ballast resistor with an external voltage regulator. So if you have changed your ignition to an HEI it could be a inline ballast resistor or a resistance type wire lowering your volts to 9.3 to accommodate a points ignition system. If not I would check into all wiring and then a new ignition switch.