Msd run-on!Has anybody experienced this?

No reason to keep the ballast resistor in the circuit with an MSD, it's just another point of failure and provides no purpose. Just jumper the connector on the side of the 4 pin ballast that used to go to the coil. No need to touch the other side at all. Then attach the blue wire that used to go to the coil + to the small red wire on the MSD.

The on/off signal to the MSD does not require much voltage and is the reason it normally works with the ballast still in the circuit. After re-wiring per above if you still get run on then you have a short some where in your electrical system that is keeping enough power to the ballast circuit to maintain the MSD in the on condition. Putting in a toggle switch will work put the short is still there and will likely manifest it self in other ways at the most inopportune time. I recommend you take the time to trouble shoot the short and fix it.