Your 71 does not have a separate ballast resistor - the wire going to the "+" side of the coil is a resistor wire, and has the correct resistance to drop the voltage to the coil. The reason for the dropped voltage is 2-fold:
1. To extend the life of the points. The points will not last long with a full 12 volts supplied continuously
2. To prevent the coil from burning up if you leave the key in the "on" position when the points are closed. In this configuration, you have a direct current path to ground from the ignitiion switch and through the coil. If there were no additional resistance in the circuit, you'd fry the coil in minutes.
As long as nobody has cut the "+" coil wire out of your wire harness, you do not need to add a ballast resistor.