Please post pictures of the front and back of your alternator so we know what you have.
Here's a thread with lot of discussion about different alternators and wiring them.
http://67-72chevytrucks.com/vboard/s...d.php?t=763180
And here is what American Auto Wire says about wiring the CS series alternators, which is probably what you have.
https://www.americanautowire.com/PDF...20IN%203.0.pdf
I will guess you probably didn't have enough resistance in the lamp wire. Normally this would be the brown wire from the cab. It has sufficient resistance to use to excite the older external regulator or 12SI or 10SI v-belt alternators. But for the CS alternators you add resistance between 35 ohms (5-Watt resistor) and 500 ohms (0.5 Watt resistor). Since the 500 ohm resistor can be much lower wattage and is therefore small and runs cooler, I think it is the best choice. In my case when I put a CS alternator in my truck I used an adapter pigtail that has just the S and L wires going to the alternator connector. The resistor is on the L wire, built-in to the adapter. It is a little more resistance than AAW mentions, 511 ohms. I went this way so I could get the alternator connector and resistor in one purchase, and I figured AC Delco and GM probably know what resistance is needed for alternators with that type of connector, so it gave me a little more confidence. On my truck, the S wire is connected to the main hot wire that goes from the battery to the cab, but it is connected at the junction near the driver side headlight, so it is a little distance from the alternator, not just a couple inches of wire to the terminal lug on the alternator. Connecting it downstream a ways is supposed to give the alternator's regulator a better sense of the voltage in the wiring harness so it can better regulate. The pigtail/adapter I used was same part number as driller35 used in that thread I mentioned earlier.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1