If the fault is ignition, read the MSD hints and tips. There is a reference to the very high voltage ionising air in the distributor. I changed my MSD to a compatible 'Streetfire' unit, which seemed to lower the voltage slightly, now no problems. If the popping is as you describe, it would suggest an ignition fault. Does it pop in the carbs or the exhaust ?
I also had a similar issue with the induction on overrun, caused by a fuel level in the float chambers that was lower than spec. Ian's darkness test is a good tip. If it's popping in the silencers, then it's probably unburnt fuel being ignited by the hot exhaust gases. The loose-piece gaskets under the inlet manifold can sometimes migrate, giving an induction leak. What do the spark plugs tell you ? Are they all the same tip colour ? All light chocolate coloured ? If there is a slight exhaust leak causing the popping to become audible, then sealing the leak will hide the problem.
Good luck !