Okay, we got this puppy whipped. I hope the attached sketch covers what is going on. Your stock 1969 setup uses an alternator with a single wire field terminal. The regulator supplies a variable voltage to one end of the field winding through the exposed terminal. The other end of the field winding is internally grounded in the alternator.

The externally regulated Denso (and I hope I got the right diagram, the plug has two wires, a dark green and a dark green with orange?) has an isolated field, both terminals are brought out through the plug. With this arrangement one end of the field is always energized at battery voltage through the first terminal and the regulator grounds the other end of the field through the second terminal to control field excitation.

So the secret is to make the Denso look like the original from a wiring standpoint. You need to ground either one of the two leads coming out of the plug (doesn't matter if it's the dark green or the dark green with orange, the field isn't polarity sensitive) and connect your original green alternator field lead to the other wire. Connect the battery terminal and you should be good to go.

Good luck.