I don't really go for all the hand wringing about "not having a hot cable," but that's OK

I don't believe in running a DEDICATED (unswitched) alternator line back, unless it's on the cold side of the switch. That is, if you kill the switch, and it leaves the alternator "hot" I don't believe this meets the intent/ spirit of the rules

The whole deal comes down to this: The alternator will keep the ignition hot, unless you kill the ignition. Since you are using an MSD, this becomes easy.

As I said, run two number 14's from front to rear, hooked to the small terminals of your switch.

Disconnect your ignition run and ballast wire at the jumper FROM the "small red" MSD power wire. This should result in NO ignition.

Now hook the small red to one of your no14's and hook the ignition junction of your "run" and ballast splice to the remaining one.

This gets back to the old wives tale of "testing alternators" where people go around pulling off a battery clamp and "if the engine runs, the alternator is good." Same deal here---the disconnect must break either the alternator circuit or the ignition circuit.

Since those small terminals are too small to carry alternator output, and since there is no field circuit on a "one wire" AND since you handily have an MSD with the "small red" enabling wire, this just became very easy.