Re: gen light on
Since you said you already replaced the alt., reg., and battery you might want to check the connections at the horn relay. Make sure they're clean and tight. If there's a wire that goes to the + battery terminal from the horn relay check that too. My '69 has a junction block on the radiator support near the battery. Also if you've added any high amp draw accessories, like a high power stereo with a power amp, your alternator might not be able to keep up with it at idle.
69 RS 355 Tremec 5-Speed 4.11's