removing ballast resistor WIRING, MSD, switched 12v source,

I think you are overthinking it.
With my HEI ignitions removing the start and run wires back to the original harness works well and then the two connected together to run the Ignition system.
One or the other always has power to it whenever the key in on, and they don't interfere with each other as far as being connected together.

You are right in that any switched power source would work to supply your ignition also, just want to make sure you have a good strong supply and not some 15 foot long accesory wire that ends up with low voltage under a load.

You might even consider using a relay to get a good strong source, and just use the ignition on source to trigger the relay.
This would definitly go against your clean as possible goals though.

You might want to really consider using the start/run circuit that already exists, and just connect them together like I mentioned above.