What is this ?

IGN is just switched power. We could call it RUN.
It doesn't matter much where you tap into it to power the ECU.
You could do a piggy back at the regulator, crimp a new female Packard 58 terminal with two wires,or a solder splice in the wire (with strain relief).
Piggy back some sort of second tab on the regulator is probably the easiest.
Like was said above - clean contact very important.
Also insulate with a plastic connecor or something. If there is contact with earth (ground in US/Canada English) and the key is in Run, then the alternator output and battery will discharge all the power they can down that circuit. The alternator is self protected in a way, and the battery feed on a '65 should have a fusible link, so that should be the first thing to melt - saving everything else from severe damage.

The voltage regulator is switching device. The output voltage and current flow varies. Higher voltage and more current when a stronger magnetic field is needed. Less voltage or no flow at all when a weaker field is needed.

There must be a ballast resistor or the coil will get cooked. The resistance needed depends mostly on the coil.

HEI units control current to the coil so they don't use a resistor. Capacitive discharge systems (like MSD 6, 7 etc) send power through the coil in waves (one or moer per firing). So in a different way flow though the coil primary is controlled and a resistor isn't needed. Those are the two somewhat common exceptions.