I assume you are talking about a 10SI or 12SI alternator. You looked at the wiring diagram for a '70s car and you did not and should not have seen a diode. You should have seen a resistor. In most cases it is a resistor wire, much like a ballast resistor wire but without the cloth insulation. Sometimes the wire is just listed as "Res. wire" with no symbol.
The resistor serves two purposes. It allows more current to flow to the Alt so that it begins to charge without reving the engine. The second purpose of the resistor is in case the bulb should burn out, the exciter circuit continues to function.
In the original wiring, one side of the resistor is connected to a Acc. connection on the ignition switch. On most retro-fits on older cars, you don't have an extra connection on the ignition switch.
Go to Radio Shack and get a 10 ohm, 10 watt resistor and wire it in parallel across the Gen lamp.
This will duplicate what GM did and your problem will be solved.
If you look at a wiring diagram for the older 10DN external regulated alternators and you will see the same resistor in the exciter circuit.
