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Your alt. should be putting out 14.2 volts at idle. If you have one diode out it will cause this problem.. What is you idle rpm? should be 750..

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Start with the alternator, check it at idle.

I did the Mad bypass and installed relays for the headlights and even for the AC clutch and my lights still dimmed at idle.
Took a closer look at the alternator and found it low at idle, as soon as I'd rev it some the voltage would come up.

Replaced the alternator(which may have been defective, stocker 20+ years old) with a powermaster and now I'm good at the lowest idle. It put's out 14.3 volts at a low idle and 14.8-15.1 volts at speed. My stock electronic ignition has never been happier. It's been 10 years on the first one on my 71 and 4 years on the second one on my 74 so far, both work great.

Instead of changing my amp meter to volts(not easy) i stuck this cheap volt gauge in my cig lighter.(easy)
It'll works good and I can see the voltage drop from the battery to the cig lighter. I put a good DVM on the battery and checked the voltage both places and the plug in DVM is accurate. When I'm not wanting to see it I toss it in the console.