Although it not clear to me how you have the Gen light wired in on your car with the 3G alternator, when the Gen light is ‘flickering’ in typically means your alternator is not putting out enough voltage compared to the battery voltage.  If it were on solid, it typically means the alternator is not providing any output.  Suggest checking the voltage at the battery which the key ‘off’, then with the engine running.

I used the same conversion on my '67.  There's a resistor or two in the box and some wiring.  Pretty simple as I recall.  Its more for looks and so the factory dash light works, but its not totally empty. 

You're correct.  Suggest using the 'post' on the rear of the ignition switch, it is powered when the iginition switch is in 'On' or 'Acc'. 

I have a starter/alternator shop where I live, I've been using them for years. I had my F-250 7.3 alternators refreshed with 250,000 miles on them, the shop recommended not to replace the original voltage regulators on them because they said everything available is junk. When they did the 3 g on my Stang they said the same thing. So far I haven't had any problems. When I go to the salvage yard I grab what I find!