I remember the same thread. I think that the MSD was getting feedback through the alternator warning light - something that wouldn't happen until roughtly ERA 640. Could be wrong, though.
I don't remember experiencing runon on any cars we've built.
Generally, run on comes from two things: High idle speed and a hot spot in the combustion chamber.
Keep the idle as low as possible. Sometimes when the engine is dynamically timed, it results in too little advance at idle. You should have 8-12 degrees advance at idle. If you have less, it may be necessary to change the distributor advance limits.