Like Vikki says, if you have a one wire alternator installed you need an adapter harness if you still have the sensor plug in the loop. It's the 2 wire plug at the back of the old alternator that sends the signal to the GEN light to light up and go off.
If it's an aftermarket harness made for a one wire alternator then there "should" be instructions regarding this. If it's a stock harness and the plug is just "hanging out" you need to get the adapter for it.
I personally like the externally regulated on my 68', up at the top of the rad support keeps it nice and cool. Had the original regulator until about 6 months ago or so when she gave up!
