6Al2 Box 12v Keyed source

Easy. And it's been covered here a zillion times. You have ONLY ONE source of power for "run" in the ignition bay AND THAT GOES DEAD during cranking That is the (normally dark blue) IGN1 "run" wire

That is "made up" by the bypass / brown / IGN2 circuit which went originally to the coil+ side of the ballast This is hot during cranking and it's job is to send hot "full battery" voltage to the coil during "start"

So to run a system without a ballast resistor, you must jumper the IGN1 and IGN2 together and use that junction as your source

You can actually use the old coil+ wire to feed the "small red" which acts like a relay/ trigger wire. Jumper around the old ballast and away you go

CAVEAT. MSD is completely different than older ignition. Make CERTAIN there is NOTHING attached to the coil, NOTHING AT ALL except for the two wires from the MSD box. No radio cap, and no tach wire.